In this work, we analyze the Gassmann’s well-known equation combined with Eshellby-Walsh ellipsoidal inclusion theory to consider the pore geometry to inverse porosity which has been proved that the compressional wave velocity influenced by the pore geometry greatly in carbonate reservoirs. And we get an approximate form to Gassmann’s equation to propose a new method to inverse porosity suitable for carbonate reservoirs after analyzing the suitable approximate and replacement of the coefficients. We present a working flow for inversing porosity, and also the calculating method of all parameters for the inversion is proposed. We use a real survey data to prove the applicability of the method and get conclusion that the method is suitable for predicting the porosity for carbonate reservoirs with high accuracy. This method is suitable for predicting the porosity of carbonate reservoirs because the bulk modulus of carbonate rock is much bigger than other rocks like sandstones and so on, especially in the deep depth, where can result in high-pressure compaction, and where the compressibility of dry rock is much smaller than the rock pore fluid’s.