The flow mechanism in the shale reservoir involves the seepage, the diffusion and the desorption. A productivity formula for the shale gas is obtained under the stable condition with considerations of both the Knudsen diffusion and the desorption. With this formula, a productivity equation for the fractured vertical wells is established by the method of percolating resistance, with consideration both of the desorption and the diffusion. According to the formula derived and the production examples, the influence of the diffusion coefficient, the half-length of fractures and the fracture conductivity on the well gas productivity is studied. It is shown that the gas production increases with the increase of the diffusion efficiency. The greater the fracture conductivity, the larger the gas production will be. When the fracture conductivity adds up to 0.12 μm^{2}·cm, the amplification of the gas production decreases significantly. The contribution of the free gas accounts for 85%～90%. So the free gas has a great contribution on the productivity. The model provides a theoretical basis for the production prediction and the optimization of the development index.