Windbreak mechanism under different shrub cover conditions was quantitatively studied by carrying out a field investigation in southwestern marginal zone of the Mu Us Sandy Land, northwestern China. Artemisia ordosica and Salix psammophila shrub land were taken as the object of this study. Bare sand was set as control and the wind velocities at 0.1, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 m height upon research grounds were recorded. By doing statistical analysis of wind velocity measurements, drawing wind profile, calculating the wind profile equation, boundary layer thickness, aerodynamic roughness, we got the conclusion that both A. ordosica and S. psammophila shrub land decreased wind velocity of boundary layer much more significantly than bare land, and this effect decreased with wind velocity increasing. Aerodynamic roughness of shrub land decreased logarithmically with wind velocity increasing. When wind velocity was high, boundary layer displacement thickness would reflect the decrease of wind velocity at observation height more realistically than aerodynamic roughness. Analyzing from the air momentum, the loss of air momentum on the surface covered with vegetation was relatively stable. While the loss of air momentum on bare sand would decrease as wind velocity increasing.