Abstract: In software system life cycle, the evolution of the requirements is unavoidable, that is a constantly evolving and stepwise perfecting process.In order to reduce the software systemfailure which caused by the deficient requirements and change of requirements, the evolution course of the requirements should be finished at steady state.A nonlinear equation is proposed todescribe this process, the initial research shows that the process can be kept stable and the requirements can be changed if the requirements rate of change can be in a steady area, if the requirements changing rate is too large, the process may be in chaos and the change is hard.The result shows that we can get a stable evolution process as long as in an appropriate range of changing rate.