Abstract Background: The purpose of this study was to analyze the consistency of bladder filling before and during radiotherapy for pelvic cancer patients. Methods: Before radiotherapy, 105 patients followed a strict bladder protocol of consuming 540mL of water immediately after emptying the bladder. Bladderscan device BVI 9400 was used after emptying bladder and measured every 30 minutes until the bladder volume achieve 400mL. When emptying, half an hour and 1 hour after drinking, chief complaint of urgency, the actual bladder volume and corresponding time were described as: U 0 , t 0 ; U 0.5 , t 0.5 ; U 1 , t 1 ; U t , t; U T , T. During radiotherapy, 18 patients were randomly selected from 105 patients. They were instructed to keep the same pattern of suppressing urination during the following radiotherapy. The chief complaint of urgency during radiotherapy was observed and recorded. The relative bladder volume was as close as possible to (50%,155%). Results: Before radiotherapy, patients were no statistically difference between U 1 (P=0.177) and U T (P=0.052) in the Immobilization, Simulation CT scan, X-ray Simulation. Bladder volume was concentrated at 298-526mL. There was no statistical difference between U t and U T in X-ray Simulation(P=0.198). Patients emptied bladder volume for 75.2±49.9min, U t =331.2±140.3mL. During radiotherapy, 18 (18/105) patients received a total of 450 bladder volume (18*25). U P and U T were statisticall different (P<0.05), and the difference was 17.81%. The overall relative bladder volume was negatively correlated with the number of radiotherapy (r = -0.5726, p = 0.0028) and the consistency rate was 82.89% (373/450). 15 patients (15/18) passed, and the consistency rate was 96% (360/375). 3 patients (3/18) failed, and the consistency rate was 17.33% (13/75). The consistency rate had no linear correlations with age (P = 0.2741). Conclusions: U 1 was consistent in the Immobilization, Simulation CT scan, X-ray Simulation and during inter-fraction radiotherapy. This indicates that the consistency range is reasonable. Relative bladder volume between (50%,155%) can determine whether the bladder filling of patients with pelvic cancer is consistent with the planned bladder volume before radiotherapy.