Aiming at the current, electric-magnetic torque and speed oscillation problems during the soft-starting of induction motor controlled by thyristors, the soft-starting simulation model is built. Based on the simulation study of influencing factors and causes of oscillation during the soft-starting, and further analysis on voltage and mechanical properties of motor starting, a scheme of power-factor angle closed-loop control based on gradient method is proposed. The trigger-angle of dynamic process is compensated by power-factor angle closed-loop feedback, and the calculation is simplied via gradient method. The validity of this method has been proved through simulation results, and on this basis, the soft-start experimental prototype has been developed. The feasibility of the control scheme also is verified through analyzing the experimental results.