In order to investigate the different nutritional quality of lamb at different time after slaughter, knuckle muscles in Mongolian and small-tailed Han crossbreed lamb were used as experimental material in this study. The content of moisture, total fatty acid, total amino acid and mineral content of lamb were determined after postmortem and storing at low temperature (4℃) for 7 time points (1h, 6h, 12h, 1d, 3d, 5d, 7d). The results showed that the water content of lamb at early stage of postmortem (1-6h) was significantly higher than that of 1d postmortem (P<0.05). The ratio of unsaturated fatty acid to saturated fatty acid decreased with the prolongation of postmortem time, which was closer to the recommended ratio of China Nutrition Association in the meat after 1h postmortem. Mineral element content (dry basis) decreased with the extension of postmortem, which may be due to some mineral elements in lamb lost with the loss of water. The total amino acid content was not significantly affected by the postmortem time. Therefore, based on analysis of nutrient content, the nutritional level (moisture, minerals and fatty acid) of lamb within 1-6h postmortem was relatively high.