A machine vision-based displacement monitoring system for radiotherapy is designed and implemented to address the patient’s body position movement during fractionated radiotherapy. The system places three cameras on the two sides of the radiotherapy table and the middle and upper part of the end of the bed，with active illumination provided by infrared LEDs，to capture images during radiotherapy in real time. A normalized cross correlation-based template matching algorithm is used to track markers pre-positioned on both sides and in the middle of the patient’s body membrane in real time. Field experimental results show that the system has a measurement mean error of 0.008 mm and a standard deviation of 0.028 mm，and can be used for real-time monitoring of minute patient body movement during radiotherapy.