Abstract: In order to accurately understand the downhole motion laws of the variable angel bent sub of steering drilling tool in the process of drilling, the kinematics model of the full rotary built-in variable angle bent sub was established according to the theory of spatial mechanism, and its accuracy was verified by means of the Solidworks 3D simulation. Then, motion laws of the system under different tool angles, tool face angles and working states were analyzed. In this model, the Matlab is used for simulation. The basic kinematical parameters of key parts are obtained, e.g. inner eccentric ring, outer eccentric ring and bit’s angular velocity and displacement. The kinematical characteristics of variable angle bent sub is revealed, the relationships between rotation speed and excentricity and bit’s motion trajectory, and the change laws of system parameters over the time are concluded, so as to provide the theoretical basis for the borehole trajectory control of full rotary built-in variable angle bent sub.