The responses of stem radial growth to climate are the basis of dendroclimatology. There is much research on the tree radial growth response to climate change in northwest China. However, most studies are based on statistical analysis, lacking an understanding of tree growth processes. Therefore, two Picea wilsorii trees were continuously monitored with a dendrometer in the Tulugou National Forest Reserve in the eastern Qilian Mountains from 2013 to 2015 in this study. We extracted the stem radial increment at different time scales (1 day, 7 days, 10 days) and mean climate factors. Our studies have shown that the extension of the time scale weakens the disturbance of diurnal variation in the trunk water, but the available water status is still a major limiting factor for stem radial growth. To further understand the major climatic factors affecting radial growth, we calculated the moving correlation coefficient between the daily radial growth and the daily meteorological data (temperature, precipitation, VPD) within a window of 31 days. We found that the stem radial growth was significantly positively correlated with precipitation in May and July during the three years, but the change of available water was large to the radial growth of Picea wilsorii during the three years. Thus, the relationship between the stem radial growth and precipitation in June was unstable. This may be the reason why the relationship between tree-rings and precipitation in June is not significant in some studies on the climate response of tree-rings.