Single-track freight railway currently adopts the way of departure with a fixed headway under inter-station blocking system to organize train operation, due to the limitation of line conditions, which can not meet the growing freight transport demand. In order to improve transport capacity, this paper introduces the concept of train convoy into train scheduling problem of single-track freight railway, and proposes a train operation strategy on this basis. Firstly, this paper analyzes the impact of train convoy on the train operation strategy of single-track freight railway. Secondly, on the basis of considering the constraints of the number equality between arriving trains and departing trains at the departure station, the number of arrival and departure tracks at intermediate station and running time in section, a mixed integer programming model is constructed with a goal of maximizing railway freight volume at a fixed time. Thirdly, this paper designs Harris Hawks optimization algorithm (HHO) based on characteristics of the model to solve it. Finally, the experiments based on some single-track freight railway in China are implemented to verify the effectiveness of the model and algorithm. The results show that the train operation strategy proposed in the paper can increase transport capacity by 27.05 %.