Abstract: Based on the porous medium theory and the continuum medium fluctuation theory， this paper studies the amplitude re flection and energy reflection properties of the plane P1 wave reflection on the unsaturated semi-space free boundary. Using Helm holtz decomposition theorem and specific free boundary conditions， the analytical expressions of the amplitude reflection coefficient and energy reflection coefficient of four types of reflected waves （reflection P1 wave， reflection P2 wave， reflection P3 wave， and re flection S wave） generated by the plane P1 wave are obtained， and the effects of incidence and saturation， frequency and porosity on energetic properties is analyzed. The results show that the amplitude reflection coefficient and energy reflection coefficient not only are affected by the angle of incidence， but also have significantly changed with the change of saturation， and the reflected P1 wave and reflected S wave carry the vast majority of the incident wave energy.