Objective To compare the setup errors between abdominal deep inspiration breath hold (ADIBH) guided by real-time position management (RPM) and free breathing (FB) for breast cancer patients who were treated with intensity modulated radiation therapy (IMRT) after breast-conserving surgery. Methods The data of 60 patients who underwent breast-conserving surgery for left-sided breast cancer and completed IMRT were analyzed retrospectively. Of these patients, 30 received ADIBH technique guided by RPM and 30 received FB technique. Setup errors in translational (X, Y, Z) and rotational (Rx, Ry, Rz) directions were assessed by comparing planning CT and cone-beam CT (CBCT) images for both patient groups. Results Compared with FB group (232 sets of CBCT images), ADIBH (261 sets of CBCT images) significantly reduced setup errors in the translational directions (X, Z) and rotational directions (Rx, Ry, and Rz) (Z values were 3.14, 2.42, 1.45, 1.93, 1.37, respectively; all P<0.05). In the ADIBH group, the difference in setup errors between the patients with BMI <24 kg/m² and those with BMI ≥24 kg/m² was not statistically significantly different (P≥0.05); no significant change in setup errors was detected when comparing the first treatment week with subsequent radiotherapy fractions (P≥0.05). The rotation error in the Ry direction was greater in the first treatment week than subsequent radiotherapy fractions in the FB group (Z=8.02, P=0.02). Conclusion In left-sided breast cancer patients receiving postoperative IMRT, the ADIBH technique demonstrates significantly smaller setup errors compared to FB technique, independent of BMI, thereby improving radiotherapy precision.