Fly ash cement grouts overcome the disadvantages of chemical grouts, which is poisonous, too expensive, and the gel strength is lower. It is a kind of grouts commonly used materials in grouting engineering. This paper reveals the hydration mechanism of fly ash cement according to the analyses of enthalpy and free energy. High temperature is the main reason to form fly ash activity, and fly ash with activity and cement to form stable cementation hardening body. The laboratory experiment is made on permeation grouting of small particle size sand by a self-developed testing device. The results shown that the strength and mechanical parameters of the grouted media are improved remarkably. Although the effect of grouting with thick grout- density is good, the grout-density has an influence to the spread of grouts. The penetration ability of thin grouts is strong than thick grouts. By the orthogonal design direct analysis, it is found that the influence of various factors to grouting quantity and diffusion radius of grout are decreasing in the following order: water cement ratio, grouting pressure and water segregation rate. And the regression model of grouting amount and diffusion radius with various impact factors have been put forward by test data.