Abstract: In order to analyze the effect of immersion on polyurethane-solidified sand, the micro and strength characteristics of reinforced sand after immersion are tested by NMR and unconfined compression tests. The results show that with the increase of curing agent content, T₂ spectra develop from a single peak to multiple peaks, and the amplitude area continues to increase. The amplitude intensity and area increase with the increase of immersion time. With the increase of curing agent content and immersion time, the total effective porosity increases, and the proportion of pore throat increases. With the increase of density, the proportion of small pore throat increases, and the proportion of big pore throat decreases. With the increase of curing agent content and immersion time, the more the bright spots in the NMR image of the sample section, the more the effective pores. With the increase of the density, the bright spots in the NMR image change from continuous concentration to non-dispersive state, and the macropores in the sample decrease and the micropores increase; and the strength decreases with the increase of immersion time. The strength of reinforced sand with curing agent of 1% is the highest before immersion for 24 h. When the content of curing agent is more than 2%, the strength increases with the increase of the content of curing agent. With the increase of density, the strength keeps increasing after immersion. The microcosmic variation of solidified sand soil is tested by NMR. The influences of water immersion on the microcosmic properties of polyurethane reinforced sand and the microcosmic mechanism of the macroscopic strength properties are revealed.