In view of the problem that the slurry outlet of the secondary mixer is often blocked in the engineering application of continuous mining and continuous gangue cemented filling system in Changcheng No. 6 Mine, the Fluent Euler multiphase flow model and Realizable k-ε turbulent model was used to simulate the transient flow of solid-liquid two-phase flow in mixer based on the sliding grid method. The reasons for blockage were analyzed, and optimization indicators were determined. On this basis, the effects of rotational speed, blade inclination angle, impeller diameter, and impeller number on the stirring effect were studied separately. It was found that increasing rotational speed, increasing blade inclination angle within 45 °, and appropriately increasing impeller diameter and number of impellers can help improve the mixing effect and alleviate blockage. Finally, the improvement plan for the secondary mixer was determined as a rotational speed of 75 r/min, blade inclination angle of 34 °, impeller diameter of 1.3 m, and a 5-layer impeller, which effectively reducing the two-phase flow density at the bottom of the tank and improved the fluidity of the mortar at the outlet.