The permeability of gravel soil is a key index for analysis of seepage through gravel soil and stability of slope. The gravel soil has the unique properties of homogeneity, discontinuity and so on, and it is very difficult to study. The studies of influences of gravel content, void ratio and grain composition on the permeability coefficients of the gravel soil have been carried out using a large-size permeameter. The analysis of effects of the gravel content and void ratios on the grain composition was made in the tests of seepage. The test results show that the relationship between the gravel content and the permeability coefficients fits in with an exponential function; the relationship between the void ratios and the permeability coefficients is comformable a multinomial function; and the relationship between the grain composition and the permeability coefficients is a linear one. The effects of the grain composition on the seepage are mainly the non-uniformity coefficients and characteristic particle size, and the curvature coefficients are affected slightly. Before and after seepage through the gravel soil, the changes of the grain composition are related to the hydraulic gradients. The research results can provide an available reference for getting values of the permeability coefficients and for establishing an empirical formula.