Abstract: The uncertainty and variability of requirements always affect on software life cycle，and are important factors in puzzle efficiency and quality of software development.According to the research on dynamical systems and software of complexity theory，an evolutionary model is developed，which can guide the evolution course of the software requirements and evaluate the quality of the requirements course.It shows that the evolution process of the requirements can be regarded as the normal when the consistent of requirements evolutionary process change chart and the model trajectory.The efficient and effective of this model on estimate requirements evolutionary process is illustrated by a specific example.