The sap flow velocity (SFV) of 7 individual Pinaceae schrenkiana trees in a fixed plot at the middle region of the northern slope of Tianshan Mountains were measured in July. Two nonlinearly regression models (quadratic one and power one) were established to disclose the relationship between diameter at breast height (DBH) and SFV and that between DBH and sap wood area. According to the two models, we calculated the transpiration of Pinaceae Schrenkiana stands. It is found that biggest transpiration and mean transpiration of Pinaceae. Schrenkiana stand are closely related to planting density, which can be fit well in Extrem function. According to the function, we find that the maximal transpiration hit 77.27t/hm²d when planting density increases to 1200/hm². In addition, the transpiration of Pinaceae Schrenkiana is positively related to plot size, with transpiration change slowly when plot size is beyond a threshold value. This finding can be used in transpiration study in middle region of the northern slope of Tianshan Mountains. The threshold value of plot size is location dependent as it is 4hm² in the highest or lowest altitude of researching region, and 2.56 hm² in the middle altitude.