Aiming at the problem of water loss of grout in the grouting reinforcement of fully weathered granite stratum when small pores were encountered, filtration experiments of different bentonite-superfine cement grout formulations are carried out, and the microstructure of filtration mud cakes is compared and analyzed. The results show that the content of bentonite and the morphology and Morphological distribution of colloid determine the filtration property of the grouting. Appropriate addition of ultrafine cement can increase the content of fine particles and reduce filtration by making the colloid easier to fill the gap between particles. Excessive addition of ultrafine cement leads to colloid being wrapped by fine particles, which can’t prevent filtration, the filtration increased. To study the effect of colloid on the seepage, numerical simulation was studied by using finite element method. It was found that the filtration resistance of dispersed block and large and small pores was very poor, and complete membrane was the best.