To investigate the sealing mechanism of polyurethane for tunnel leakage, a model test of tunnel leakage and polyurethane sealing is designed and conducted. The development process of external stratum erosion triggered by tunnel leakage in different strata is discussed. The following conclusions are drawn: (1) The pore water pressure decreases rapidly after tunnel leakage both in sandy and silt soils, and the water is recharged to the leakage point in the form of seepage under the action of pressure difference, resulting in the seepage erosion of the surrounding strata. The two different strata show different erosion development patterns. (2) The leakage sealing mechanism by polyurethane grouting in the sandy and silt soils is similar, and it can be divided into three phases: the first phase, the polyurethane reacts with water and releases a lot of gas to resist seepage water pressure; the second phase, the polyurethane consolidates, thus seals the leakage channel and realizes the leakage plugging effect; the third phase, the polyurethane consolidates completely, resulting in the redistribution of the external load of the tunnel structure. (3) Due to the difference in seepage erosion patterns between sandy and silt soils, the sealing effect of polyurethane varies in different strata, and the effect of grouting and sealing in silt strata is relatively more obvious.