Abstract: In this study, one representative slopes covered by the 33-year-old Larix principis-rupprechtii plantation were selected in the small watershed of Xiangshuihe within the semi-humid region of Liupan Mountains, northwest China. Sixteen stand plots were set up continuous along the slope positions of slope top. The biomass was measured in the middle of growing season in 2014. The results showed that the mean biomass on the slope was 118.59 t/hm2, with a range of 96.19 139.18 t/hm2, and the variation range of 42.99 t/hm2 and the coefficient of variation was 0.12. The variation pattern of biomass along slope positions was shown a trend of first increase then decrease, reached the maximum at the middle-upper slope(horizontal slope length is 87.71 m). The biomass existed the slope scale effect. It was found that the moving averaged biomass (Y1) increases gradually with the increasing relative horizontal length (X1) of slope section from slope top. The relationship to express this scale effect on the shady slope is Y1=23.004X13-60.834X12+31.786X1+123.43(R2=0.84).The slope average of biomass increased 6.12 t/hm2 per 100 m increase in the slope length on the slope. The ratio of plot biomass to the slope average (Y2) varies nonlinearly along the relative horizontal distance of plots from slope top (X1), with the relation expressed as Y2=1.7226X13-2.8445X12+1.0338X1+1.0001(R2=0.57), This relation can be used to calculate the slope mean biomass from the biomass measured at certain slope position. The slope variation of biomass mentioned above is mainly caused by the comprehensive effect of solar radiationand duration of sunshine with the change of elevation along the slope.