Abstract: In order to improve the performance of coal mine filling slurry before and after solidification, sodium sulfate and polycarboxylic acid water reducing agent were used to prepare a composite admixture, and the effect on the working performance and mechanical properties of the filling slurry before and after solidification was explored through indoor macroscopic experiments. Its influence mechanism was analyzed in combination with indoor microscopic experiments. Experiments show that the cement consumption of 0.5wt% sodium sulfate and 0.2wt% polycarboxylic acid water-reducing agent mixed filling slurry can be reduced by 2%, the slump can be increased by 4.1 cm, the initial and final setting time can be shortened by 20 min, and the water reduction rate can be increased by 7.7%, the 3rd day and 28th day uniaxial compressive strength can increase up to 22% and 42%. The analysis shows that the early strength mechanism of sodium sulfate, the electrostatic repulsion and steric hindrance of polycarboxylic acid water-reducing agent, and the mutual promotion of the two are the main reasons that affect the working performance of the filling slurry before curing. The needle-like product ettringite and the white fibrous substance calcium silicate hydrate are the main substances that affect the mechanical properties of the filling paste after solidification, and the formation of the two improves the uniaxial compressive strength of the filling paste.