Abstract: Treatment of tumor with high intensity focused ultrasound(HIFU)is a new method developed in recent years, which has received increasing attention.Presently available tumor therapy instruments treat patients by immerging the target into water and with the transducer deployed beneath the target and emitting upward.This is not only inconvenient, but also impractical in treating certain special body parts such as cervix.In this paper, a HIFU tumor therapy instrument is designed by placing the transducer into a water bag on top of the patient and transmitting downward.This requires higher precision, speed and reliability of the control system.A DSP based board is developed as an independent frontend controller.All the functions needed by the instrument including control of step-motors, pulse-coders and ultrasound power amplifiers, and computation of real-time control-strategy are integrated on the board.The DSP board communicates with the host PC via a standard RS232 interface, and is connected with peripherals such as step-motors, pulse-coders, etc., via photoelectric isolation and voltage conversion circuits.The instrument has passed both laboratory test and vitro-test in hospital with satisfactory results.