Objective:To observe the efficacy and safety of combined lienal polypeptide injection therapy in the treatment of Mycoplasma pneumoniae pneumonia（MPP）in children aged 3 to 14 years old in multiple clinical centers.Methods:A randomized，controlled，multi-center clinical study design was adopted.A total of 240 hospitalized children aged 3 to 14 years old with MPP from 7 hospitals from September 1，2023 to January 31，2024 were included.According to the severity of pneumonia，they were divided into the mild MPP group with 80 cases and the severe MPP/refractory MPP（SMPP/RMPP）group with 160 cases，and then randomly divided into the control group and the experimental group at a ratio of 1 ∶1，using the random number table method.After screening，subjects entered a treatment period of 5 to 7 days.The control group was treated with azithromycin，while the experimental group was treated with azithromycin plus lienal polypeptide injection .The recovery of lung CT，length of hospital stay，duration of fever，cough score，whether mild cases developed into severe or refractory cases，duration of hormone use，use of intravenous immunoglobulin（IVIG），bronchoscopy treatment，and immune function were observed between the two groups to evaluate the efficacy of lienal polypeptide injection.Adverse events after medication，vital signs，blood routine，urine routine，liver function，myocardial enzymes，renal function，and electrocardiogram were observed to evaluate the safety. Results:A total of 231 subjects have completed the trial in the 7 hospitals，including 118 cases in the experimental group and 113 cases in the control group.Main observation index：the rate of lung CT aggravation in the experimental group was lower than that in the control group（2.6% vs 15.3%， P<0.01），and the difference was statistically significant.Secondary indexes：there were no statistically significant differences in the length of hospital stay，duration of fever，cough score，duration of hormone use，whether IVIG treatment was used，the number of bronchoscopy treatment cases，and immunoglobulin between the two groups（all P>0.05）.However，the rate of cases of plastic bronchitis（PB）found under bronchoscopy in the experimental group was lower than that in the control group（0 vs 18.8%， P=0.03），and the difference was statistically significant.Among the mild MPP（72 cases），there were no statistically significant differences in the length of hospital stay，duration of fever，cough score，duration of hormone use，whether IVIG treatment was used，the number of bronchoscopy treatment cases，and the improvement rate of lung CT between the two groups（all P>0.05）.However，compared with the control group，the rate of cases developing into SMPP/RMPP in the experimental group was less（24.3% vs 48.6%， P=0.03），and the difference in IgG before and after treatment was small［0.53（-0.04，1.18）g/L vs 1.33（0.48，2.25）g/L， P=0.01］.Among the SMPP/RMPP cases（159 cases），the rate of cases of PB found under bronchoscopy in the experimental group was less than that in the control group（0 vs 20%， P=0.04），and the rate of cases with aggravated lung CT in the experimental group was less than that in the control group（1.3% vs 19.5%， P<0.01），and the improvement rate of lung CT in the experimental group was higher than that in the control group（88.8% vs 75.3%， P=0.03），with statistically significant differences.There were no statistically significant differences in the length of hospital stay，duration of fever，cough score，duration of hormone use，whether IVIG treatment was used，the number of bronchoscopy treatment cases，and immunoglobulin between the two groups（all P>0.05）.Two cases in the experimental group developed rashes，which improved after the drug was discontinued.There were no serious adverse reactions such as abnormal vital signs like dyspnea and cyanosis due to the use of lienal polypeptide injection.There were no obvious changes in blood routine，liver function，myocardial enzymes，renal function，electrocardiogram，and urine routine values before and after medication compared with the baseline. Conclusion:The combined use of lienal polypeptide injection in the treatment of MPP in children can reduce the probability of the transformation from mild cases to SMPP/RMPP，reduce the rate of aggravation of the image findings，promote the absorption of lung inflammation，reduce the rate of PB found under bronchoscopy，and has good safety.