Abstract: An error propagation model of the ultrasound image-guided robot for liver cancer coagulation therapy is established,which consists of an ultrasound imaging machine,a navigation software subsystem,a position tracking unit and a needle-driven robot.Through 3D ultrasound reconstruction,registration between pre-operative model and intra-operative physical body and coordinate-transformation from position tracking unit to robot,the robot system firstly converts the target of tumor into robot coordinate frame,and then it controls the robot to move to the designated position for therapy.The surgery procedure is analyzed firstly,and the error sources are pointed out.Secondly,error propagation model of target mapping is established by differential matrix of homogeneous transformation,and the model is validated by simulation results.At last,precision test for the system is taken,and the result shows that total error of the system is no more than 5 ram,which can meet the need of coagulation therapy for liver cancer and improve the efficacy of liver treatment effectively.