Objective:To investigate the feasibility and adverse reactions of simultaneous integrated boost (SIB) in volumetric modulated arc therapy (VMAT) for early breast cancer after breast-conserving surgery.Methods:A total of 67 patients with early breast cancer after breast-conserving surgery at Zhongshan People's Hospital from September 2019 to May 2021 were enrolled. All patients received VMAT-SIB to the whole breast and tumor bed. The total breast dose and tumor bed dose were 40.5Gy/15 times every 3 weeks and 48 Gy/15 every 3 weeks respectively. The exposure dose of organs at risk and acute adverse reactions of radiotherapy were evaluated.Results:The average doses of planning target volume of the whole brease (PTV WB) and planning target volume of the boost(PTV BOOST) were (42.0±2.1) Gy and (49.9±0.8) Gy, respectively. The V 95% and V 105% of PTV WB and PTV BOOST were (98.8±1.2)% and (31.4±11.3)%, (99.8±0.6)% and (22.9±10.2)%, respectively. The V 5Gy, V 20Gy, V 30Gy and mean dose (D mean) of ipsilateral lung were (52.4±12.0)%, (15.3±4.5)%, (6.7±2.2)% and (11.0±2.4) Gy respectively. The V 18Gy, V 40Gy and D mean of heart were 3.80% (0.48%,9.60%), 0 (0,0.16%) and (6.7±2.1) Gy respectively. At the end of radiotherapy, 19 patients (29%) of all 67 patients had no acute skin toxicity, 41 patients (61%) showed radiation erythema, 5 patients (7%) had radioactive dry peeling and 2 patients (3%) had wet peeling mainly located in the nipple, areola area and breast folds. None of the patients had grade 3-4 acute skin reactions. Breast swelling and breast pain were found respectively in 9 patients (13%) and 7 patients (10%). No local recurrence or distant metastases were observed during the follow-up period. The 2-year disease-free survival rate was 100%. Conclusions:VMAT combined with SIB is feasible in the treatment of early breast cancer. The adverse reactions of radiotherapy are mild and well tolerated.