The nutrient release experiment of polyurethane coated urea (PCU) was carried out in pure water at 25 ℃. With the release of urea, the structural variation of polyurethane coating was investigated by Fourier transform infrared spectroscopy (FTIR), then a series of curves were collated and compared so as to better reflect the relation between diffusion rate of urea and coating structure. It was found that when the nutrient was released by 50% wt, new absorption peaks at 3 435, 3 342, 1 671, 1 621, 1 448 and 1 159 cm-1 appear in the FTIR spectra of polyurethane coating, moreover, the height of these absorption peaks was increased gradually when the nutrient was released by 70% wt, more importantly, these new absorption peaks are consistent with the characteristic absorption peaks of urea fertilizer, the spectra of urea were mainly characterized by bands at 3 440, 3 346, 1 672, 1 621 and 1 461 cm-1. The results show that the IR spectra variation was caused by the content of urea, existing in the polyurethane coating, and was increased gradually, The more the urea content, the greater the swelling degree of the polyurethane coating. The swelling of polyurethane coating leads to the pores size change, and release rate is increased, so the “S” pattern curve of the nitrogen accumulative release is formed.