Abstract: This paper applied the theory of self-organization to the study of department economic system of the mid-hierarchy to establish a non-linear differential equation model system to reflect the relation between the supply and the demand. This model system consists of four basic equations: (1) the Logistic equation corrected for the price factor; (2) the Cobb-Douglas equation corrected for the price factor; (3) the supply - demand equlibrium equation comprised the depreciation form and the depreciation factor; (4) the probability conservation equation using as its random variable RA, a ratio of the commodily volumn adjusted by marketing to that adjusted by planning.A set of simulation solutions are obtained by the SD method through this model system. The adjustment problem of branch structure for the mechanical-electronic industry system in Henan province was analysed. An operable satisfactory result was achieved.