Objective To investigate the relationship between cystatin C(CysC)/prealbumin(PAB)ratio and lung function and chronic obstructive pulmonary disease(COPD)assessment test(CAT)score in patients with COPD.Methods A total of 216 COPD patients admitted to the hospital from February 2021 to February 2024 were selected as the study group,and 150 healthy subjects were selected as the control group during the same period.CysC and PAB levels were detected by immunoturbidimetry,CysC/PAB ratio was calculated,lung function was detected and CAT scores were recorded.Pearson correlation was used to analyze the rela-tionship between CysC/PAB ratio,lung function and CAT score in COPD patients.Results The CysC and CysC/PAB ratio in the study group was significantly higher than that in the control group,and the level of PAB was significantly lower than that in the control group,with statistical significance(P＜0.05).The forced expiratory volume in the first second as a percentage of predicted value(FEV1％pred),the percentage of pre-dicted forced vital capacity(FVC％pred)and the ratio of the first second forced expiratory volume to forced vital capacity(FEV1/FVC)of the study group were lower than those of the control group,and the CAT score of the study group was higher than that of the control group,with statistical significance(P＜0.05).The Cy-sC/PAB ratio and CAT score of COPD patients with the global initiative for chronic obstructive lung disease(GOLD)grade 4 were significantly higher than those with GOLD grade 3,GOLD grade 2 and GOLD grade 1,and the difference was statistically significant(P＜0.05).The ratio of CysC/PAB in very severely affected pa-tients was higher than that in severely affected,moderately affected and mildly affected patients,while the ra-tio of FEV1％pred,FVC％pred and FEV1/FVC was lower than that in severely affected,moderately affected and mildly affected patients,and the difference was statistically significant(P＜0.05).In COPD patients,Cy-sC/PAB ratio was negatively correlated with FEV1％pred,FVC％pred,FEV1/FVC,and positively correlated with CAT score(P＜0.05).Conclusion CysC/PAB ratio is highly expressed in COPD patients,and its level is related to lung function and CAT score,which can reflect the degree of disease and quality of life in COPD patients to a certain extent.