In order to meet the requirements of velocity control performance of servo system for K mirror in the 2 m telescope system, a new active disturbance rejection-controller(ADRC) based on adaptive control law parameter was proposed. Firstly, the second-order linear extended state observer based on first-order velocity model was designed; And then, in order to improve the dynamic and steady-state performance of speed-loop, proportion controller′s parameter based on regression analysis method was designed into a function and it will be automatically adjusted with the variation of input speed. Finally, experiment system based on K mirror servo control was set up, the experiment was performed under the excitation of step signal. Actual results show that compared with the PI and ADRC controller, the regulation time for 0.001 (°)/s step response is reduced from 7.3 s, 3.2 s to 0.9 s. The overshoot for 10(°)/s step response is reduced from 8%, 62% to 0; The best disturbance inhibition ability is increased 23 dB among the low and middle frequency band, satisfying the high accuracy speed control performance requirements of K mirror servo system.