Orthogonal modelling is currently of great importance in a number of disciplines, including the exploration and development of unconventional oil and gas reservoirs, nuclear waste storage, carbon dioxide sequestration and groundwater hydrology. The most common approach to modelling mixed fluid saturation is to replace an ensemble of multiphase fluids with an equivalent fluid. This paper investigates the effect of the equivalent fluid model on the elastic properties of a saturated porous medium based on a rock physical model of the effect of intersecting and non-intersecting fractures on the elastic properties of the rock physical model.This paper focuses on the case of partial saturation of a gas-water two-phase mixed fluid and introduces the dimensionless parameter q to quantify the degree of saturation of such a mixed fluid. This approach builds on single fluid theory and further explores the complex interactions of two-phase fluids and their physical manifestations. The parameter q is employed to characterise the pressure variations within the fluid, as well as the fluid mixing. It represents an effective method whose value affects the stiffness of the equivalent fluid. The use of the equivalent fluid and parameter q allows for the investigation of the impact of water and gas components on the elastic properties, anisotropy parameters, and the generation and variation of P-wave and S-wave velocities in the rock physical model. This enables a more accurate simulation of the properties of fractured reservoir media.