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Pore pressure prediction plays an important role in hydraulic fracturing. The pore pressure in shale cannot be directly measured but to be inferred by the normal velocity trend, so methods based on the effective stress theory are dedicated to establishing a function between seismic interval velocity and pore pressure. Among them the mostly used is Eaton's equation. However, how to precisely quantify the state of compaction remains unsolved. In this study, the AVO/AVA simultaneous inversion was introduced to estimate P-velocity. According to the exponential relationship between the pore pressure and the ratio of normal compaction acoustic transit time to the measured, three methods based on Eaton's equation were proposed for shale gas pore pressure, respectively. The fitting method avoids the estimation of the normal compaction trend (NCT) by the fitted nonlinear relationship between borehole-side pore pressure (inferred by the Eaton's equation) and P-velocity. The direct calculation method directly estimates the NCT with the linear trend line (LTL). Whereas the model-based calculation method constructs the NCT model using the NCT transit time well logs obtained by the LTL. The result shows that the approach of estimating the NCT impacts the pore pressure both on the accuracy and the horizontal continuity significantly, which implicates that the constraint between traces must be taken into account when computing the NCT, as well as lithology.