In this study, we present a comprehensive experimental and modeling study on six shale samples to investigate the effect of shale composition and pressure on methane diffusivity. By correlating shale composition, pressure, and methane diffusivity, it was found that both macro- and micropore diffusivity decreased with increasing pressure, while total organic carbon content had little effect on macropore diffusivity and negatively affected micropore diffusivity. This phenomenon may be a result of nonporous organic matter (OM) in transitional shale acting as solid material instead of porous media, occupying micropore volume, prolonging gas diffusion length, and increasing diffusion resistance. Clay minerals with connected microstructures positively affect micropore diffusivity while negatively affecting macropore diffusivity, which may be a result of the filling of macropores with clay particles, blocking gas diffusion pathways, and increasing gas diffusion resistance. Brittle minerals have a positive effect on macropore diffusivity and a negative effect on micropore diffusivity, which is similar to their effects on macro- and micropore volume, respectively. Moreover, the effect degree of shale composition and pressure on methane diffusivity changed over methane absorbing and diffusing, which may be related to the fact that methane diffusion occurs in different pores associated with brittle minerals, clay, or OM at different pressure steps.