Shale gas flow is controlled by multiple mechanisms and multi-stage hydraulic fracturing often creates complex fracture networks. It is very challenging to incorporate various migration mechanisms of shale gas and accurately simulate the complex fracture networks in shale gas reservoirs. In this work, we present a semi-analytical model to study the pressure behavior in shale gas reservoirs. Multiple migration mechanisms are considered, which contain diffusion of dissolved gas in kerogen bulk, desorption from the surface of kerogen bulk and clay particles, slippage flow simultaneously from both organic pores and inorganic pores into natural fracture system and Darcy flow in natural fractures and hydraulic fracture network system. Line source function, Laplace transformation and the principle of superposition are employed to analytically describe flow behavior in reservoir system and a numerical discrete method is adopted to simulate the complex fracture networks. Main fractures and secondary fractures within the fracture networks are divided into elements with equal length. Gauss elimination method and Stehfest inversion algorithm are employed to calculate the pressure responses. We perform three case studies which contain multistage transverse fracture, orthogonal fracture network and un-orthogonal fracture network. Type curves are plotted and possible flow regimes are identified. With its rapid computational speed, yet capturing the physics of shale gas flow, this semi-analytical approach provides an efficient tool for people from multiple disciplines to perform pressure transient analysis of complex fracture networks in shale gas reservoirs.