Based on the study of medium pressure differential inpipe robot structure, and using the technology of computer flow dynamics (CFD) for analyzing the flow field of the drive speed control unit of the robot, the pressure field and velocity field were gotten under the drive speed control unit in different working conditions. The influence of drive speed control unit and tube wall annular gaps on the moving velocity of the robot was analyzed. The result shows that the upstream flow field of the drive speed unit does not change obviously with the increase of the annular gap.The velocity of the gap decreases with the increase of the gap and the range of the wake region decreases gradually. Along with the increase of the annular gap gradually, the pressure in the highpressure area becomes smaller gradually, while the pressure in the negative pressure area also rises slightly, and the difference of the pressure between the two ends of the drive unit becomes smaller gradually.