Objective To investigate a simple and easy method to maintain a stable urine volume and consistent bladder filling during immobilization, CT scanning, plan designing, and radiotherapy using an ultrasonic bladder capacity scanner (BS). Methods A total of 66 patients with pelvic tumor who were admitted to our hospital and received radiotherapy from 2013 to 2014 were enrolled and required to drink 500 ml water after emptying the bladder. BS was used to measure bladder capacity at four time points. Before radiotherapy, 62 patients with pelvic tumor were randomly divided into two groups in a single-blind trial. Both groups were asked to empty the bladder, drink 500 ml water, and wait for a subjective feeling of urgency of urination. The 42 patients in group 1 were asked to hold urine, and bladder capacity was measured and urine volume was recorded when patients complained of “urgency of urination”. The patients in group 2 were told that urine volume should reach the standard in plan designing, and BS was used to determine whether the standard was reached. Results The time from bladder emptying to the subjective feeling of urgency of urination ranged from 0.5 to 1.5 hours;when the patients had a subjective feeling of urgency of urination, the urine volume ranged from 30 to 500 ml. In group 1, when the patients had a subjective feeling of urgency of urination, the difference between the bladder capacity measured and that in plan designing was 30.6%, while in group 2, the difference was 12.6%. Conclusions The use of BS in monitoring bladder capacity helps patients to develop a stable feeling of urgency of urination and reach the bladder capacity in plan designing and thus maintain a consistent degree of bladder filling.