The output voltage of the three-phase bridge half-controlled rectifier circuit is often used as the exciting voltage in the excitation system of small and medium-sized turbo-generator sets. However, it is easy to be out of control and affect the normal operation of generator set. In this paper, the causes of the runaway of the three-phase bridge half-controlled rectifier circuit are studied in depth, and the operation state of the circuit after the runaway under the loss of the firing pulse is analyzed. Based on a suppression method with a parallel freewheeling diode, an improved circuit with parallel resistance and capacitance is proposed to ensure the reliability turn-off of the thyristor. Finally, the simulation experiment based on Matlab/Simulink verifies the feasibility so as to provide theoretical support for improving the operation stability of generator set.