Estimating stimulated reservoir volume (SRV) in shale gas reservoirs with high accuracy has been of more concern to oil and gas industries. However, current SRV prediction methods are of limited use for characterizing critical flow mechanisms. To make more accurate prediction of SRV in shale gas reservoirs, multiple mechanisms cannot be ignored. In this paper, we develop a novel analytical model to accurately estimate the volume of SRV in shale gas reserviors by incorporating both Knudsen diffusion of bulk gas and surface diffusion of adsorbed gas directly into the model. Depending on flow discrepancies from conventional reservoirs, the modified pseudo-pressure equation to account for these critical transport mechanisms are further constructed. Predicted values of SRV by using this new model are in fair agreement with values from the CMG simulation. Compared with related research, it is the first time that both Knudsen diffusion of bulk gas and surface diffusion of adsorbed gas are taken into consdertion to analyze and estimate the volume of SRV in shale gas reservoirs. A clear workflow for implementation of this method is presented. Compared with the common numerical reservoir simulators, this approach is easier to setup and less data-intensive.