In order to optimize the accelerating performance of the diffuser of a certain axial-flow generator, the influence of its main structural parameters on its acceleration performance is studied. By using the method of numerical simulation and experimental design, the length of the expansion section, the length of the middle section and the opening angle are chosen as the main factors. The L9(34) orthogonal table is selected to obtain 9 sets of schemes. In the fluid simulation software, the numerical model of the diffuser under different structural parameters was established. Orthogonal experiment was carried out on the line-diffuser with the accelerating performance of the diffuser taken as the evaluation index. Through the range analysis method, the influence of the main structural parameters on the acceleration performance is explored, and the order of the influence of the acceleration performance is obtained. Finally, the optimal parameter combination is obtained when the length of the expansion section is 1 000 mm, length of the middle section is 800 mm, and the angle is 15°. This study provides a reference for the design of the diffuser of axial flow generators.