Objective To observe the daily bladder and bowel preparation of patients with prostate cancer by cone-beam computed tomography (CBCT), and analyze its impact on the precise implementation of radiotherapy for prostate cancer and side effects. Methods We retrospectively analyzed 38 patients with prostate cancer who underwent volumetric modulated arc therapy. The number of radiation fractions for each patient ranged from 25 to 35. A CBCT scan was performed before each daily radiation therapy, and the number of scans for each patient ranged from 25 to 40. Setup errors were adjusted to ensure that the tumor was targeted and the rectum wall was not in the high-dose target area of the prostate. There were 93 instances where treatment could not be successfully implemented and re-preparation and re-scanning were required. We calculated the success rate of treatment and setup errors, compared radiotherapist-adjusted error values under different bladder and bowel preparation conditions, and recorded radiotherapy-related side effects. Results The success rate of treatment in the 38 patients was (92.14 ± 5.25)%. Among the 93 instances of seriously inadequate preparation, 48.4% were due to insufficient bladder filling, and 30.1% were due to intestinal bloating. Radiotherapy side effects were negatively correlated with the success rate of treatment (r = −0.393, P = 0.015). When bladder filling was sufficient, there were no significant differences in radiotherapist-adjusted error values in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions between adequate and inadequate bowel preparation (P > 0.05). When the bladder was moderately or insufficiently filled, there were significant differences in radiotherapist-adjusted error values in the LR, SI, and AP directions between adequate and inadequate bowel preparation (P < 0.05). Conclusion Insufficient bladder filling and intestinal bloating are the main factors influencing the successful implementation of radiotherapy for prostate cancer. When the bladder is sufficiently filled, bowel preparation does not affect prostate position change.