Abstract: According to the characteristics of the low strength reinforcement of the coal mining broken roof under the direct cover of the caving zone, based on the influence of fly ash on the performance of the cement slurry, the indoor static test method was used to optimize the slurry with different cement fly ash ratios, and the optimal strength and liquidity ratio were obtained. The results showed that the compressive strength and fluidity of the slurry decreased with the decrease of solid ratio (the increase of fly ash content), while the setting time and the rate of stone increase with it. The early strength of the hardened body was low, and the later strength increased greatly. When the solid ratio was 2∶8, the physical and chemical reaction between cement and fly ash was good. The fluidity performance was good as a whole. The strength of stone body was above 1 MPa and the setting time was longer. The research results had achieved good application effect in the ground grouting reinforcement project under the direct cover of the caving zone in 7228 working face of Xutuan Mine.