This article mainly introduced a cost effective mechanical positioning device in MRI-PHIFU(Magnetic Resonance Imaging- Phased High-Intensity Focused Ultrasound) system. The ultimate goal was to implement the control of the mechanical positioning accurately, which based on the ultrasonic transducer of the PHIFU system, In the passage, a design of precision mechanical device was proposed, including design of ultrasonic motor control system and implementation process of positioning the ultrasonic transducer, and the accuracy of the entire system was analyzed.