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.