Abstract: The quasi-saturated soil is defined as a soil with entrapped air and saturation close to 100%. A series of one-dimensional ponded water infiltration tests are designed and conducted in laboratory. The influence factors of entrapped air content in the quasi-saturated soil, such as soil types, dry density and the initial water content, are studied. The influence mechanisms are analyzed and discussed. The results show that the volume of entrapped air decreases as dry density increase; the escape of entrapped air is more difficult when the initial water content increases above a critical water content; the escape ratio of entrapped air in sand is smaller than that in clay because of the relatively longer time to reach stable state of infiltration for the clay. Some physical phenomena in real soil infiltration are shown. The results provide test data for better understanding of soil infiltration process and numerical modelling.