Objective To explore the preventive effectiveness of early physiotherapy on arm lymphedema after modified radical mastectomy for breast cancer. Methods A total of 206 patients who underwent modified radical mastectomy for breast cancer in The First Affiliated Hospital of Henan University from June 2014 to June 2016, enrolled in this randomized controlled clinical trial. Then these patients were randomly divided into intervention group and control group equally. Patients in the control group received routine treatment, and the patients in the intervention group began to use the air pressure pump combined with the microwave physiotherapy on the second day after the radical surgery. The incidences of limb lymphedema in 6 months and 1 year after operation between the 2 groups were compared, and the influencing factors of arm lymphedema were explored. Results The clinical data of 195 patients were analyzed at end, including 99 patients of the intervention group and 96 patients of the control group. ① There were statistical significance in the incidences of arm lymphedema in 6 months and 1 year after operation between the 2 groups (P<0.05), that incidences of arm lymphedema in the intervention group were both lower than those of the control group at the2 time points [6 months after operation: 2.0% (2/99)vs. 9.4% (9/96); 1 year after operation: 5.1% (5/99) vs. 17.7% (17/96)]. ② The results of non-conditional logistic regression analysis shown that, age (OR=1.45, P=0.008), tumor location (OR=1.72, P<0.001), TNM stage (OR=2.01, P=0.033), the number of invasive axillary lymph nodes (OR=1.15, P=0.005), and postoperative radiotherapy (OR=1.23, P=0.016) were the influencing factors of arm lymphedema after modified radical mastectomy for breast cancer, patients with age older than 60 years, tumor position at the outside area, stage Ⅲ of TNM, the number of invasive axillary lymph nodes >5, and patients received radiotherapy after operation had high risk of arm lymphedema. Conclusion Early physiotherapy can effectively prevent the occurrence of arm lymphedema after modified radical mastectomy for breast cancer, and early physiotherapy should be performed for patients with high risk of arm lymphedema.