For the multi-stage fracturing in horizontal shale gas well, the stimulated reservoir volume (SRV) is a pivotal parameter to determine the fracturing performance. However, SRV estimation remains a longstanding challenge in shale gas reservoir due to its complex forming mechanism, most of the current SRV estimation methods are either expensive or time-consuming. In this paper, a 3D mathematical model of SRV is established by simulating the four key processes—multiple hydraulic fractures propagation, reservoir pressure lifting, formation stress variation and natural fractures failure—during each fracturing stage in shale gas reservoir. This model considers the interference effect of multiple fractures, and subdivides the SRV into shear-SRV and tensile-SRV according to the failure type of natural fractures network. In order to validate the feasibility and reliability of the model, it was first implemented to a pilot well in the FL gas field in southwest China to estimate a SRV that matches well with the microseismic signals monitored onsite. Then, this model was applied to FL gas field on a large scale, the application results show that although most horizontal wells have been effectively fractured with desired SRV, there are still unstimulated regions left between neighboring wells, so it needs to drill infill wells in current well pattern and reduce the well spacing in the future. This research explores an efficient method to estimate the SRV without high cost or complicated process, provides the theoretical basis and guidelines for pre-fracturing design and post-fracturing evaluation in shale gas reservoir.