Objective To apply real-time transperineal ultrasound ( TPUS) to monitor the intra-fractional prostate motion,collect and analyze the data of the prostate motion,aiming to provide evidence for the optimization of the target area and plan of radiotherapy for prostate cancer. Methods TPUS is a non-invasive monitoring technique that utilizes an automatic scanning ultrasound probe to dynamically monitor and correct the motion of organs during radiotherapy. In this study, TPUS was utilized to collect 1588 intra-fractional data of 70 patients with prostate cancer. Prior to each cycle of radiotherapy,CBCT was adopted to correct the errors between intra-factional data by using VMAT. During radiotherapy, real-time ( once per second) ultrasound images were acquired to monitor the 3D motion of the prostate on the x (left+,right-),y (cranial+, caudal-), and z ( abdominal+, dorsal-) axes, emphasizing the analysis of 4D motion of the prostate. Results All patients successfully completed the treatment and data collection. The median time of effective monitoring per faction was 179 seconds (132-286 seconds). During 95% of the monitoring time,the distance of prostate motion was 2. 22 mm in the left direction,2. 17 mm in the right,2. 08 mm in the cranial, 1. 98 mm in the caudal,2. 44 mm in the abdominal and 2. 97 mm in the dorsal direction,respectively. In the x,y and z axes,the percentage of time in which the distance of prostate motion less than 1 mm among the total time was 83. 07%,85. 46% and 78. 27%,respectively,whereas 97. 70%,97. 87% and 96. 45% for＜3 mm, respectively. Conclusions TPUS is a non-invasive real-time monitoring technique,which can detect the 4D motion of the prostate during radiotherapy. By using VMAT,the range of prostate motion is relatively small, and the motion range is less than 3 mm in each direction within 95% of the time.