Water pollution has gradually become one of the prominent problems restricting the harmonious development of urban ecosystem. At present, the development of efficient wastewater treatment technology has important practical significance to alleviate this problem. Wastewater treatment reactor is the core equipment of wastewater treatment, and the internal flow characteristics affected by its structure have an important impact on water treatment efficiency. Fluidization reactor has become a common sewage treatment equipment because of its high heat and mass transfer rate. In view of the liquid phase back mixing and uneven liquid-solid mixing still exist in fluidized reactor for wastewater treatment. Based on the mobile biofilm wastewater treatment technology and the concept of maximizing the space utilization of integrated wastewater treatment equipment, a rectangular fluidized reactor with slit hole conical distribution plate was designed. The orifice area of the distribution plate gradually increased from the center of the reactor to the outside, and the Euler-Euler multiphase flow model and RNG k-ε turbulence model were used for numerical simulation of the fluidized reactor with a conical distribution plate with slit holes. The effect of the distribution plate structure on the distribution of particles in the reactor was studied by the arrangement of the holes and the cone angle of the slit hole conical distribution plate. The results showed that the conical distribution plate with slit holes can solve the problems of liquid-phase backmixing and uneven liquid-solid mixing, form multiple ring core flows in the reactor, improve the uniformity of particle distribution, and strengthen the liquid-solid mixing. Based on the comprehensive evaluation of particle volume fraction, flow rate, and bed density standard difference, the optimal hole distribution mode of the slit conical distribution plate was determined to be perpendicular to the central axis, and the optimal fluidization effect of the reactor appeared under the cone angle of 120°.