A criterion is proposed to judge if the axes of the emergent beam and the telescope of coaxial Mie lidar are in the same line. Then the auto collimation system of coaxial Mie lidar is designed, in which the control target aims at the maximization of the criterion, the two-dimension electric driving mirror is used as actuator, and the variable step size regulation algorithm is used. The experiment results show that the system can realize auto collimation regulation with high precision and high speed, and the regulation accuracy is up to 0.05 mrad. The effectiveness of collimation criterion and regulation algorithm is verified. The system is benificial for realizing automation and unattended operation of lidar system.