The sap flow and environmental factors, including temperature, water content and water potential of soil, were continuously measured by using an auto-data collection system at the upper and lower slopes of Toona ciliata var. pubescens plantation in July to October, 2012 to investigate the relationships between the sap flow and environmental factors. The results showed that, the trunk sap flow velocity of the two positions both presented a typical single-peak curve, with high values in the daytime and low values in the nighttime. The average sap flow of the lower slope was significantly higher than that of the upper slope. Soil temperature of the upper slope was significantly higher than that of the lower slope, and soil water content and water potential were vice versa. Soil water content and water potential were the leading environmental factors affecting the trunk sap flow velocity at the lower slope, while soil temperature and water potential were the main environmental factors at the upper slope.