The work aims to analyze the internal flow field characteristics of vertical hydraulic pulpers to provide a theoretical basis for improving the pulping effect and the design and production of pulpers. A numerical simulation analysis of the internal flow field of a hydraulic pulper was carried out with Ansys Fluent software. The control variable method was used to study the impact of the number of blades, the rotor clearance from the bottom, and the number of spoiler plates on the flow field characteristics. The characteristics of the flow field were evaluated by the velocity field, pressure field and rotor power consumption. The numerical simulation results showed that the fluid flow in the vertical hydraulic pulper was a typical axial flow mode. The number of blades, the rotor clearance from the bottom and the number of spoiler plates had a greater impact on the static pressure of the flow field and the power consumption of the rotor, while the impact on the velocity of the flow field was relatively small. When the number of rotor blades was 2, the rotor clearance from the bottom was 10 mm, and the number of spoilers was 3, the power consumption of the pulper structure was reduced by 8.5% compared with the original structure. The internal fluid flow velocity was also faster than that of the original structure, while the pulping efficiency was improved. The internal flow characteristics of the hydraulic pulper were obtained. The research results provide a theoretical basis and reference for upgrading of vertical hydraulic pulpers and promote the development of the paper packaging industry.