The photoaging of polyurethane coating based on TDI-TMP and N3390 was conducted though UV exposure. The properties of studied polyurethane coatings, such as ﬁlm thickness loss, gloss loss, discoloration and ultraviolet absorbance were examined at some aging intervals. In addition, scanning electronic microscopy (SEM) and Fourier transform infra-red spectroscopy (FTIR) were used to measure the morphology and chemical change of coatings before and after exposed in a UV lamp chamber. It was found that the addition of N3390 had a signiﬁcant effect on the discoloration of polyurethane coating, and coating without N3390 had greater degradation. The above results suggested that a mixed curing agent system with a mass ratio of TDI-TMP to N3390 between 5:5 and 3:7 be a possible substitute of pure N3390 for weather resistance PU coatings.