To study the climate response mechanism of tree radial growth using physiological models is important for understanding the physiological mechanism of tree growth, predicting changes of forest ecosystem in climate change scenarios, and providing suggestions for forest protection and management. Based on the tree-ring records of Picea crassifolia in the middle of the Qilian Mountains, the Vaganov-Shashkin model was used to simulate the radial growth of Picea crassifolia. The physiological mechanism of the radial growth of Picea crassifolia was also discussed. The results showed that precipitation was the main factor affecting the radial growth of Picea crassifolia in low altitude area of middle Qilian Mountains. Precipitation from May to August directly affected the radial growth of Picea crassifolia, and precipitation in last September also affected the radial growth of Picea crassifolia. It is suggested that the irrigation amount should be increased appropriately in May-August in afforestation of Picea crassifolia.