The study was conducted on Robinia pseudoacacia and Pinus tabuliformis forestlands of different slope aspects in Caijiachuan watershed, a loess region of western Shanxi Province. Based on the located observation of soil moisture at 0 -200 cm soil layer of fixed sample plots in R. pseudoacacia and P. tabuliformis forestlands from 2005 to 2012, optimal partition for ordinal specimen method was used to analyze the annual and interannual changes of soil moisture in R. pseudoacacia and P. tabuliformis forestlands, and the influence of precipitation distribution on annual and interannual changes of soil moisture in R. pseudoacacia and P. tabuliformis forestlands was discussed. The results showed that: 1) the annual dynamics of soil moisture in R. pseudoacacia and P. tabuliformis forestlands in the study area could be divided into four stages: stable stage, fluctuating stage, accumulative stage and consumption stage; in sunny slopes, the soil moisture condition of R. pseudoacacia forestland was similar to that of P. tabuliformis forestland, while in shady slopes the soil moisture condition of R. pseudoacacia forestland was the best; 2) the soil moisture deficit amount of P. tabuliformis forestland was less than that of R. pseudoacacia forestland in drought years, and during the interphase period of drought years and normal years, the soil moisture of P. tabuliformis forestland recovered faster than R. pseudoacacia forestland; 3) the annual precipitation distribution evenness was inversely proportional to the annual fluctuation range of soil moisture in R. pseudoacacia and P. tabuliformis forestlands, while the interannual change of soil moisture content in R. pseudoacacia and P. tabuliformis forestlands lagged behind the interannual change of precipitation. The suggestion of planting P. tabuliformis in sunny slopes and R. pseudoacacia in shady slopes was proposed for the vegetation rehabilitation in the loess region of western Shanxi Province.