Abstract: This paper, based on a way experimented in the open and windtunnel, puts forward a new index, X_i (a ratio of the average gap between the crown of one tree and the other to average height of the belts), which can evaluate the effect of shelterbelts. Research has shown that X_i is useful for planning and designing shelterbelts and can also judge the sheltering effect of belts growing in the fields. The protected character and mechanism of three types of belts are also studied in the paper. The results are briefly summarized as follows:1. The influence range of different types of belts extends about 4H in the vertical range and 50H in the horizontal range. The distance where wind speed reductions are still at least 20 per cent occurs 18.5H behind belts with a permeability of 35 per cent. The value of wind speed in the lee of belts is closely related with turbulence (r=-0.94).2. X_i is a important index for expressing the effect of the belts on air flow as well as permeability. The optimum X_i for each row in the belts having 1-row to 8-rows was given in the paper.