Multistage fracturing of horizontal well technique is applied to develop shale gas reservoir economically and effectively，during which a complex fracture network is formed in the formation. Therefore，it is crucial to quantitatively describe the complex fracture network to achieve an accurate prediction of gas well productivity. In order to analyze early flowback water production data and to get reservoir stimulation information，a mathematic percolation model for water phase was proposed at the constant production stage of gas wells on the hypothesis of both radical flow and linear flow in single fracture considering the influence of gas flow from matrix to fracture on water production. Combined with material balance equation，the bottom hole pressure was obtained，and according to the Duhamel’s principle，the percolation model could be extended to the blowing off stage with a variable production rate. Analysis indicates that when the internal depletion flow appears in the whole fracture system，the relation curve between normalized water rate and material balance time in the loglog coordinate is a straight line with a slope of -1. Characteristic line diagnosis could be used to calculate the effective fracture volume and fracture shape parameters. The methods is verified by numerical simulation，and the effective fracture volume is calculated combined with field cases. Results show that the flowback data collected in the blowing off stage cannot be neglected，otherwise the effective fracture volume is relatively smaller. The proposed method provides the technical support for the evaluation of fracturing performance of shale gas wells with the use of flowback data.