By taking the Northern Shaanxi Loess Plateau as a study area,the topographic wetness index is calculated by using the single flow direction algorithm and multiple flow direction algorithm. In calculation，it is considered that the flow direction algorithm will have a significant impact on the result of topographic wetness index. The correlation of soil moisture and the topographic wetness index are analyzed based on the data which consist of sampled soil moisture data and topographic wetness index data. The quality of topo-graphic wetness index data calculated by using the single flow direction algorithm and multiple flow direction algorithm are then compared. The best flow direction algorithm to soil moisture simulation is put forward.