In 2017 and 2020, China successfully performed two hydrate production tests near Sites GMGS3-W17 and GMGS6-SH2 in the northern South China Sea. Two tests revealed that large amounts of free gas are enriched below the bottom simulating reflection (BSR). However, hydrate saturations at this depth derived from chlorinity data and a pressure core range from 27% to 41%. Well log data in this interval are characterized by low P-wave velocity and high resistivity. Therefore, resistivity-derived hydrate saturations are much higher than P-wave velocity-derived values. This discrepancy occurs because free gas would induce a sharp decrease in P-wave velocity. Currently, the gas–water mixing model is commonly employed to resolve this discrepancy. To improve accuracy, we develop a novel rock physics model of multiphase mixing coexistence including solid phase, hydrate–gas phase, and gas–water phase. The model is applicable to both isotropic and anisotropic hydrate reservoirs. To validate its accuracy, the model and well log data are utilized to predict saturations at six sites worldwide where hydrates and free gas coexist. The predicted hydrate and gas saturations are consistent with those derived from pressure cores and chlorinity data. This suggests that the model can be robustly used for quantitative characterization of hydrate when coexisting with free gas. Therefore, our model will provide an accurate method for the resource predictions of hydrate and free gas in such coexisting sediments, especially those below the conventional BSR. It will also facilitate the understanding of the fluid distributions in such a complex three-phase coexistence system in nature.