Objective:To evaluate the difference between conventional fractionation radiotherapy (CRT) and stereotactic body radiation therapy (SBRT) in lung cancer.Methods:A model was constructed based on convolution operation and finite element method, simulating the dose delivered to circulating lymphocytes (CL) in radiotherapy. The model was trained on a training group ( n=3) and validated in an independent validation group between SBRT ( n=10) and CRT ( n=10). The peak cumulative dose of circulating lymphocyte (PCDC) was compared between the two schemes, and the effect of different PTV volumes and treatment time on the cumulative dose was also analyzed. The correlation between PCDC and CL change value was discussed. Results:In the training group, PCDC with CRT to CL were 1.26 Gy,1.79 Gy, 2.54 Gy in PTV of 38 cm 3, 63 cm 3 and 114 cm 3, and PCDC with SBRT to CL were 0.84 Gy, 1.22 Gy, 1.55 Gy in PTV of 38 cm 3, 63 cm 3, 114 cm 3, respectively. PCDC gap of SBRT to CL was decreased by 0.42 Gy, 0.57 Gy, 0.99 Gy, respectively. In the validation group, the lymphocyte change value in SBRT and CRT were (1.10±0.64)×10 9/L and (0.42±0.48)×10 9/L, and there was significant difference ( P=0.015). PCDC in SBRT and CRT were 3.56 (2.79, 3.82) Gy and 1.24 (0.697, 1.73) Gy, and there was significant difference ( P<0.001). There was a positive correlation between lymphocyte change value and PCDC ( r=0.455, P<0.05). Conclusions:SBRT, compared to CRT, will lead to lower PCDC and CL lymphocyte change value, which may cause a greater impact on the difference of PCDC along with the enlargement of PTV. CRT and large PTV volume may cause more significant effect upon the body immune function.