In order to comprehensively evaluate the function of water conservation of litters in Robinia pseudoacacia forests and provide the theoretical basis for the function-oriented stand improvement in Gully Region on Loess Plateau, five stand densities (475, 900, 1 575, 1 850, 2 525 plants/hm²) were chosen as subjects in this study at Caijiachuan watershed in Ji County of Shanxi province from July to August, 2017. The comprehensive coordinate method was applied to evaluate the function of water conservation of litters in R. pseudoacacia forests. Results showed that the litters of R. pseudoacacia forests could become effective fully on water conservation when the stand density was 1 575 plants/hm². However, the ability of the litter to play the function of water conservation was significantly reduced when the forest density was too high (2 525 plants/hm²) or too low (475 plants/hm²). The forest density should be controlled at 1 500 plants/hm² so that its litters could play fully the function of water conservation in R. pseudoacacia forests. Therefore, it was advised that the existing forest of R. pseudoacacia should be improved accordance to this forest density.