We studied the vertical change characteristics of soil moisture of Robinia pseudoacacia plantations grown in different water ecological zones of the Loess Plateau,sampling at Ansai County of Shaanxi Province and Jingchuan County of Gansu Province,which lie in semi-arid hilly and gully region separately.The results showed that:(1)there were objective different under the different water ecological zones of the soil moisture vertical changing regularities.The peak of soil moisture appeared in the 20-30 cm deep in Jingchuan,then the soil depth was gradually increased,while the soil moisture was gradually decreased,And below the 200 cm deep layer tended to be stable(about 11%).The peak of soil moisture appeared in the 60 cm deep in Ansai,and below the 220 cm deep layer tended to be stable(about 5.5%).It showed that the soil moisture of Jingchuan is higher than that of Ansai.The effects of rainfall and forest roots on soil moisture in Ansai were greater than in Jingchuan,both on extent and depth.The deep soil moisture tended to a stable value,did not affecte by rainfall,roots consumption water etc.(2)The balanced model to describe precipitation infiltrating in the soil,it can good fit the vertical changes of soil moisture in the two zones(Jingchuan and Ansai).By establishing functions of the parameter t(month),a dynamic model was established to describe the changes of soil moisture with the time and the soil horizon depth.It was validated the dynamic model can be used to accurately calculate the soil moisture in different water ecological zones of black locust forest in the Loess Plateau,and has a higher application value.