Abstract: A three-term AVO approximation equation of fluid-bulk modulus K_f, bulk modulus μ and porosity ϕ is derived based on the four-term approximate equation of K_f-f_m-ρ-ϕ proposed by Yin X Y et al. (2014) and the assumption of Zhang et al. (2014). Accuracy analysis and model testing show that:when the incident angle reaches 27°, the error of the reflection coefficient between the approximate equation and the Zoeppritz equation is 0.015, and the accuracy meets the inversion requirements. Furthermore, the errors of inversion result and the original curve are within the allowable range under conditions of noise and no noise, and the stability also meets the inversion requirements. Therefore, the AVO approximate equation based on K_f-μ-ϕ has certain feasibility under theoretical conditions. In the application of actual data, the inversion results are basically consistent with the corresponding results of the original curve, which lays the foundation for the inversion of the fluid volume modulus K_f and the reservoir prediction and fluid identification.