Accurate tracking of train speed curve is the key to stability, accuracy and safety of automatic freight train operation system. The large time delay of freight train speed tracking system leads to the decrease of control accuracy and the increase of energy consumption. To solve this problem, the response time and command transmission delay characteristics of traction braking system are studied, the lag model of train speed tracking control is constructed. The difference operator is used to deal with the lag term in the train dynamic model, the optimal predictive speed tracking controller is constructed. The simulation and comparison experiments show that the freight train speed tracking controller based on optimal predictive control has the characteristics of fast response, small speed error and high safety.