Wind erosion is mainly a phenomenon of arid and semiarid areas, and the porosity of shelterbelts is an important quantitative index for estimating windbreak structure. However, there appears to be considerable variation in the optimal degree of porosity. With the example of an oasis–desert ecotone in Junggar Basin, the physical mechanisms responsible for wind erosion were used to investigate the optimal porosity of shelterbelts for wind speed reduction and a mathematical model relating porosity of shelterbelts to relative wind speed was constructed. The results show significant correlation among total shelterbelt porosity, crown area, average crown height, and average clear bole height (r2 = 0.968, p = 0.000). A sharp inflection point was found near a porosity of 0.35 based on the emission mechanisms. Both emission mechanisms and the mathematical model showed that the optimal porosity of shelterbelts is between 0.35 and 0.37, confirming the results of previous research.