Economic threshold should have dynamic characteristics.It may vary from years to years,even in same field.This paper develops a system models for determining economic threshold.They are consist of four models: 1.temperature m-odel,2.population dynamic models,3.control cost model,4.product yield model. Then economic threshold,at which marginal revenue equals marginal cost, will be determined by computer simulation.User can input different parameters according to different situations. The output of this compu...