The servo control of robot based on the encoder on the joint has a sample structure, but the actual response of every joint has a small difference with the encode value, so the precise position of the end of the robot is difficult to determine. For this reason, in many situations, the precise motion of the end of the robot arm was expected to get directly in certain methods from external position, so as to realize the full closeloop servo control of the robot, however, due to the coordinate transform concerned, the traditional method was not able to be applied to it directly based on homogeneous coordinate transform matrix of the servo joints. A mathematical method is given for using, which can easily deduce all kinds of coordinate transform matrix, and give the direct kinematics of full closeloop servo control system and its research method.