To optimize structure of agriculture

forestry and animal husban dry of agricultural system and to predict their advances in the future，a dynamic simulation modelon the agricultural system in Tongwei county is developed according to the systematic dynamic theory by Forrester. Tongwei county located in hilly region of Loess Plateau is a poor region，thus it is important to adjust the internal composition of the agricultural system to raise agricultural productivity in the county. After the existent conditions and

problems of the agricultural system in the county being diagnosed

the systematic network diagrams

including crop planting

forestry

animal husbandry

population

fund

etc. are made. The systermatic simulating equations，including 18 level ones

32 rate ones

126 auxiliary ones and 127 parameters

are programmed by maens of DYNAMO language. The simulating equation model with several feedback loop is able to function automatically and control artificially by personcomputer conversation in IBM PC/XT computer. Once the model is run

all optimination values and predicting values are expressed in diagram or table.