Objective The aim of this study was to search for a better individualized clinical radiotherapy scheme for breast cancer patients after modified radical mastectomy by comparing the dosimetric data on the radiotherapeutic techniques of three-dimensional conformal radiation therapy (3D-CRT) and volumetric modulated arc therapy (VMAT). Methods This retrospective study included 77 cases of breast cancer treated by modified radical mastectomy followed by radiotherapy in Hebei General Hospital from November 2015 to February 2019. According to the radiotherapeutic techniques used, we divided the patients into a 3D-CRT (n = 54) and a VMAT group (n = 23), Using the dose-volume histogram, we evaluated the dose parameters of the clinical target volume (CTV) and the organs at risk (OAR). Results Compared with the patients in the 3D-CRT group, those in the VMAT group showed significantly decreased V110% ([12.14 ± 14.03]% vs [7.18 ± 6.36]%, P < 0.05), increased conformity index (0.62 ± 0.11 vs 0.66 ± 0.09, P < 0.05), reduced ipsilateral lung dose ([1308.42 ± 276.49] vs [1114.34 ± 233.71] cGy, P < 0.05), V5 (P < 0.05) and V10 (P < 0.05), and elevated contralateral breast dose (P < 0.05) and V5 in those with left breast cancer ([63.49 ± 17.49]% vs [76.97 ± 11.81]%, P = 0.028). Conclusion VMAT can effectively reduce the average dose and the dose in the low-dose area of the ipsilateral lung of the breast cancer patient after modified radical mastectomy. VMAT and 3D-CRT each have its own advantages for specific patients.