In order to reduce the failure of software system caused by demand defects and demand changes, the evolution process of requirements should be completed in a stable state. A nonlinear equation is established to describe this process. The parameters of the equation are mainly determined by the demand change rate, which can be divided into stable region and unstable region. If A is in a stable region, the demand can be completely evolved in a stable way, and the change of demand will not affect the final evolution result. In addition, if A is in an unstable region, the evolution process is unstable or chaotic, and the evolution result is abnormal. In this case, it is impossible to determine all subsystems. Because a small change in initial demand in evolution will cause great changes. We use the above theory to analyze the home telephone service management system and get a stable evolution process. The results show that the actual evolution trajectory is consistent with the theoretical trajectory. Therefore, we should adopt an appropriate demand change rate to get a stable demand evolution process and make the demand change.