Abstract: Based on the lots of investigation on farmland shelterbelts,we used digital camera to take pictures of shelterbelt and apply CIASsoftware to improve "digital image processing" for determining shelterbelt porosity so as to tho-roughly analyze the rule of porosity in farmland shelterbelts. Compared with the old digital image processing,the improved method possesses high accuracy,and it is more economical and more convenient to apply. The relationship between porosity of shelterbelts (β) and relative clear bole height (x) can be expressed as β=0.672x^0.469(r=0.9851,r_0.01= 0.4073,0≤β≤1) in the leaf period; and β=1.0681x^0.4304(r=0.9763,r 001= 0.4073,0≤β≤1) in the defoliation period. β varies with the stand age (a),taking the form of parabola,which can be expressed as β=0.0009a²-0.0364a+0.6828(0≤β≤1). Porosity at cross-section of shelterbelt can be used as an auxiliary index to evaluate the structure of belts. The calculation method of shelterbelt porosity of mixed shelterbelt can be expressed as β=(β₁n₁+β₂n₂)/(n₁+n₂). Among the popular,willow and elm pure shelterbelts with same disposition,popular shelterbelt structure is the best,with β_PPP<β_SSS<β_UUU,while the triangle disposition is better than the rectangle one among the same tree species. UPPU and SPPSof mixed structure have the optimal effect in popular-willow-elm mixed shelterbelts.