A volume measurement algorithm based on edge detection was introduced in this paper. The algorithm was applied to the portable system for measuring bladder volume. In the system, the three-dimensional ultrasound probe performed the scanning with the axial angle of 180 degree. Twelve axial sections of a bladder with the same scanning interval were obtained and used to edge extraction. The group of bladder edges formed a three-dimensional bladder body and the integral method of spatial volume was used to calculate the bladder volume.The average error of the algorithm in clinical diagnosis experiment was within 12%.