Based on the lots of investigation on farmland shelterbelts, we used digital camera to take pictures of shelterbelt and apply CIAS software to improve "digital image processing" for determining shelterbelt porosity so as to thoroughly 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 (beta) and relative clear bole height (x) can be expressed as beta = 0.672x0.469 (r = 0.9851, r0.01 = 0.4073, 0 < or = beta < or = 1) in the leaf period; and beta = 1.0681x0.4304 (r = 0.9763, r0.01 = 0.4073, 0 < or = beta < or = 1) in the defoliation period. beta varies with the stand age (a), taking the form of parabola, which can be expressed as beta = 0.0009a2 - 0.0364a + 0.6828 (0 < or = beta < or = 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 beta = (beta 1n1 + beta 2n2)/(n1 + n2). Among the popular, willow and elm pure shelterbelts with same disposition, popular shelterbelt structure is the best, with beta PPP < beta SSS < beta UUU, while the triangle disposition is better than the rectangle one among the same tree species. UPPU and SPPS of mixed structure have the optimal effect in popular-willow-elm mixed shelterbelts.