The light-trigged thyristor valve monitoring system of EHVDC returns the valve-side threshold voltage signal to the valve control system in real time. The valve control system integrates this signal and the grid-side dispatching information for dispatching control and protection. Several abnormal events of return of thyristor voltage monitoring unit （TVM） occur in a certain station in China. In order to investigate the cause，the valve monitoring system is analyzed，several shortcomings are found in the TVM pulse width control circuit. This paper introduces the architecture of TVM and pulse width control circuit，analyzes the principles of three kinds of pulse width control circuits，which are positive zero-crossing voltage monitoring （POS），negative zero-crossing voltage monitoring （NEG） and BOD over-voltage monitoring （BOD），and gives the optimization scheme. In the experimental verification，the experimental results of the original circuit and the optimized circuit are compared under the condition of the effective value of 380 V and the pulse voltage of 7 kV，respectively，which fully verifies the effectiveness of the circuit optimization. It provides technical support and safety guarantee for the later upgrade and transformation of the valve monitoring system of light-trigged thyristor converter station at home and abroad.