本文利用物元分析法建立了瓢虫和棉蚜的生态系统，找出了两个不相容因素（经济效益和生态效益）的相容区间，并在此区间上建立了总效益功能函数。在两因素地位平等的条件下，求出了使总效益功能值最大的最优瓢蚜结构（即棉蚜最优生物防治指标）。对两因素侧重度不同的情形进行了一般性讨论。结果较理想，并具有一定的理论和实用价值。 In this paper, we established the ecosystem of ladybugs and cotton aphids using matter-element analysis, found out the compatible interval of two incompatible factors (economic and ecological benefits), and established the total benefit function in this interval. Under the condition of equality of two factors, the optimal structure of A. calamus (ie, the optimum biological control index of cotton aphid) was found out which made the total benefit function value the largest. A general discussion was made on the differences between the two factors. The result is ideal, and has certain theoretical and practical value.