Diesel engine is a complex nonlinear system. In view of the uncertainty of the model in the process of linearization of diesel engine model and the insufficient robustness of the existing PID controller, the robust control theory is applied to the speed control of diesel engine. Firstly, the quasi-steady state model of diesel engine is locally linearized to establish its state space model. Then, by analyzing the robust stability under friction torque uncertainty, the diesel engine speed H_∞ controller is established, and the control law is finally obtained by solving the LMI method. Finally, a simulation experiment is designed in the MATLAB/Simulink environment. The simulation results show that the designed speed controller has strong model fault tolerance, good resistance to external disturbance, and the speed regulation is obviously better than the traditional PID speed controller.