Abstract: In order to study the injectability of the advanced inhibiting coal seam and the effect of inhibitor liquid, taking 3 upper coal seam in Nantun Mine as an example, we carried out the pressure water absorption test of raw coal sample in 9 kinds of inhibitor fluid based on the test and analysis of the injectability of coal seam. Combined pore distribution, we derived the minimum pore diameter of coal with different inhibitor fluids, and calculated the implantation amount of tonnage coal inhibitor. The research shows that the type and concentration of the inhibitor will have an impact on the injection effect, and the injection effect of MgCl₂ with the concentration of 25% is the best. The minimum pore diameter allowed to be injected into the coal body is 0.08 μm, and the injection volume of tonnage coal inhibitor is 3.80 kg. MgCl₂ with optimal concentration of 25% can be used to carry out advanced inhibition in the coal seam.