Abstract: Based on the analysis on MRI (magnetic resonance imaging)-guided surgical robot design requirements, a robotic system for transperineal prostate brachytherapy is designed. The robot system with 5 DOFs is actuated by MR-compatible pneumatic cylinders and ultrasonic motor, and needle posture adjustment and insertion action can be achieved automatically. By system dynamics analysis of the robot, the type selection of driving cylinder is accomplished. Finally, MR-compatibility experiment and needle insertion precision experiment are completed to verify that the robotic system can meet MR-compatibility requirement, and the precision of the needle insertion is 0.91mm.