Abstract: In this paper, the author took the intercropped fields of forest (Taxodium ascedens Brongn) and cereal crops as analytic target, which is located in Han Jiang County of Jiangsu Province. The shade effect of upper trees on the crops was discussed and some numeric bases were provided for the reasonable arrangement of intercropping density and full utilization of light and heat resources.The height and azimuth of the sun in every other hour during the day in Yang-shou district were computed for a year by use of the traditional formulas. The geometric characteristics of the Taxodium ascedens Brongns canopy, the properties of the crops (mainly rice) under the tree to make use of the solar energy, and the time for repeated shading by individual Taxodium ascedens Brongn as well as the reasonable density of afforestation in which there would be no repeated shade effects between lines and rows of 10-year Taxodium ascedens Brongn forest in different ecologic conditions were discussed.A systematic programme (BASIC Ⅱ) to compute the apparent motion orbit of the sun in any area (including the solar declination, sunshine time, the height and azimuth of the sun in evevy othev hour, etc.) and an approximative geometric method to measure the canopy structure of Taxodium ascedens Brongn were provided.