In the process of shale gas exploitation, it is difficult to simulate the real shale with a large number of micro-cavities because fluid cavities are involved, and the most important problem is the transition at the interface of the Lagrangian grid and the Euler grid. In response to this problem, a solid element model with fluid was established. The surface force balance condition at the fluid-solid interface was introduced, its modified complementary energy functional was obtained, and a new stress hybrid element with fluid was derived. By comparison with the common commercial finite element software Marc, the effectiveness of the model was verified. The equivalent elastic modulus of the model was studied in the condition when the shape, the volume fraction, the spatial distribution position, and the average radius were changed separately. The results show that when the volume fraction remains unchanged, the equivalent elastic modulus increases as the radius of the fluid-containing hole increases, while different shapes have little effect on the equivalent elastic modulus. When the volume fraction rises, the equivalent elastic modulus of the model greatly decreases. When the angle increases, the equivalent elastic modulus gradually decreases, and the decreasing amplitude drops slightly. The model has stress concentration at the fluid-solid interface, and the concentration direction is on the upper and lower sides of the hole.