Abstract: The property of the soil in free field is an important influencing factor for the free field seismic response. In the existing studies the soil at local site is supposed to be the single-phase medium or saturated porous medium. But in fact, it consists of solid phase, liquid phase and gas phase sometimes, and presents the porous and multi-phase characteristics. The researches on the wave characteristics of unsaturated soils are still at the initial stage. The analysis about the effects of saturation on free-field responses of site is also limited under the condition of S_r≥90%. To analyze the effects of saturation on free-field responses of site when S_r<90%, the free field model for unsaturated soils is established, and the wave fields in the free field of unsaturated soils under the P-wave incidence are analyzed. According to the boundary conditions, the undetermined coefficients of the waves are determined, and then the analytical solutions to the free site seismic ground motion of unsaturated soils are gained in the case of P-wave incidence. On the basis of analytical solutions, the effects of saturation on the free field seismic responses under different incident angles, wave frequencies, soil rigidities and porosities are analyzed. Some general rules of the seismic responses of the free site of unsaturated soils are obtained.