This paper reviews the studies both at home and abroad of the gas flow mechanisms in shale gas reservoirs. The mathematical description methods, the influencing factors, the applicability and the disadvantages of the desorption, the diffusion and the seepage are analyzed. It is shown that the gas flow in the shale reservoir includes desorption, diffusion and seepage. The Langmuir isothermal adsorption law could describe the shale gas desorption accurately, but further researches are needed for the law to describe multi-component gas desorption. The Fick second diffusion law describes the gas diffusion in the matrix effectively, but the coupling between the diffusion and the seepage needs to be further studied. The gas seepage in the natural fractures follows the generalized Darcy's law with consideration of the slippage effect, and the gas seepage in the artificial fractures follows the Forchheimer equation. The pressure-sensitive effect, the two-phase flow of gas and water, the thermal effects, and the phase transition are the additional flow mechanisms in the shale gas reservoir that need to be further studied.