Abstract: Bio-retention facilities, such as green space and rain garden participate in the natural infiltration and regulation of rainwater and surface runoff, and are important parts of the sponge city. The infiltration of rainwater plays an important role in the reduction of rainwater discharge quantity. At present, the design of soil seepage is mainly based on Darcy's law, but Darcy's law does not apply to the unsaturated soil. In the laboratory, the water head penetration test of different initial soil moisture content of the same soil was carried out. The change of initial soil moisture content was obtained by long-term natural water loss. A function relating water head penetration velocity and time was obtained and compared with the calculation result of Darcy's law. The results may provide reference for calculation in future design of soil infiltration in sponge city.