Objective To investigate the effect of positioning without CT end optical device and positioning with CT end optical device on the precision of radiotherapy placement during optical surface-guided deep inspiration breath-hold (DIBH) radiotherapy on left breast cancer. Methods A total of 60 patients diagnosed with left breast cancer were selected, 30 of them were guided to implement DIBH radiotherapy (group D0) through the optical surface monitoring system OSMS (AlignRT) after the positioning of DIBH without CT-side optical equipment; the other 30 patients were treated with Catalyst guided DIBH radiotherapy (group D1) after the location of DIBH was assisted by the Sentinel CT-side optical device. The positioning procedures of the two groups were compared, and the error data of six dimensions after CBCT registration in the first radiotherapy position of the two groups were analyzed. Results The absolute values of CBCT registration errors in x, y, z and Rx, Ry and Rz directions were (0.15±0.11), (0.17±0.12), (0.20±0.14) cm and (0.77±0.71)°, (0.60±0.60)° and (0.66±0.62)°, respectively. Group D1 was (0.13±0.08), (0.13±0.09), (0.20±0.11) cm, (0.61±0.57)°, (0.50±0.52)°, (0.61±0.46)°, respectively, and there was no significant difference in registration data between the two groups (P>0.05). Conclusion Positioning of left breast cancer DIBH radiotherapy guided by optical surface can be achieved accurately without CT-side optical device or with CT-side optical device, and there was no statistically significant difference in CBCT registration error between the two methods.