Fixed turn-off Angle control is one of the main control strategies for suppressing commutation failure in HVDC transmission systems. However, the current research on measured and predictive control cannot effectively solve the problem of insufficient response speed and control accuracy. Therefore, an improved turn-off Angle control method for suppressing subsequent commutation failure is proposed. Firstly, based on the analysis of commutation process and influencing factors of DC system, a prediction method of turn-off Angle considering voltage harmonics is proposed. Then, the predicted turn-off Angle of the subsequent commutation process is introduced into the original control logic of the DC system to improve the control strategy of the fixed turn-off Angle. Finally, the improved control method is simulated and verified based on PSCAD/EMTDC. The simulation results show that the proposed method can effectively suppress the subsequent commutation failure of the DC system, and improve the response speed and control precision of the fixed turn-off Angle control.