This study was aimed to understand the difference in forest transpiration between slope positions, and to separate the contributions of main influencing factors, for improving the accuracy of forest transpiration estimation at slope scale by up-scaling the results measured at plot, especially in semiarid regions with significant soil moisture difference along slope positions. Two plots of Larix principis-rupprechtii plantation were established, one at upper slope (less affected by terrain shading and more impacted by soil drought) and another at lower slope (more affected by terrain shading and less impacted by soil drought) on a northwest-facing slope in the semiarid area of Liupan Mountains in northwest China. The sap flow velocity ( J S , ml·cm ^-2·min ^-1) of sample trees, meteorological parameters in open field, and soil water potential in main root zone (0-60 cm) were monitored simultaneously in the growing season (from Jul. to Sep.) of 2015. Using transpiration data of selected 59 effective days after excluding the days with rainfall and data missing, the contributions to the transpiration difference between the two plots by soil water potential, sapwood area, terrain shading, and their interaction were quantitively separated using analytical method. The total transpiration of 59 effective days was 41.91 mm at the lower slope plot, slightly higher than that at the upper slope plot (37.38 mm), indicating a small difference (4.53 mm) due to the offsetting effects of multiple factors. A significant difference in the soil water potential of 0-60 cm was found between the two plots, with means of -0.240 MPa at lower slope plot and -0.582 MPa at upper slope plot. The promotion effect on the sap flow velocity at lower slope plot by its higher soil water potential increased linearly with rising soil water potential difference, with an average promotion of 0.017 ml·cm ^-2·min ^-1. The reduction effect on the sap flow velocity at lower slope plot caused by stronger terrain shading follows a parabolic equation to rising solar radiation intensity, with an average limitation of 0.003 ml·cm ^-2·min ^-1. When taking the upper slope plot as reference, the plot difference in soil water potential increased the total transpiration of 59 days at lower slope plot by 16.40 mm; while the differences in sapwood area, terrain shading, and the interaction of three factors decreased the total transpiration at lower slope plot by 6.61, 2.86, and 2.40 mm, respectively, making a net increase of 4.53 mm. Under given conditions of location, soil, climate, and vegetation in this study, the contributions of influencing factors to the stand transpiration difference between slope positions is: soil moisture (soil water potential) > stand structure (sapwood area) > solar radiation (terrain shading) > interaction of all factors. All these impacts should be considered for accurate prediction of forest transpiration at slope scale through up-scaling from measurement at plot, especially in semiarid regions.