Abstract: Using 146relative soil moisture observe stations data from 2003to 2008in the Beijing、Tianjing and HeBei area,we have established a spherical cap harmonic model of the annual trend.Spherical cap pole located in longitude 115°,latitude 38°.The radius of the spherical cap is 5degree while the maximum degree expands to 8degree.According to the spheical cap harmonic model,we calculate and analyze the characteristics of the relative soil moisture annual linear spatial distribution in 10cm 、 20cm and 50cm depth.