In the background of the 13th Five-Year Plan proposing that it is appropriate to reduce the rate of social insurance, this paper calculates the optimal contribution rate of basic pension insurance system for urban residents in the five types of fertility context using a general equilibrium model from the perspective of social welfare maximization, and analyzes the economic effect of the reduction in social pooling contributions. It comes to the conclusions as follows:firstly, the optimal social pooling contribution rate goes down as the extension of the retirement age and increases with the rise in population growth rate; as the percentage of eligible women having a second child ranges from 0%-100%, the optimal contribution rate decline to a level of 19.18%-19.63% under a retirement age of 60 years old, and to a level of 10.77%-11.64% under a retirement age of 65 years old. Secondly, besides fertility policy and retirement age, the optimal contribution rate is also sensitive to physical capital output flexibility, personal subjective utility discount factor, and social discount factor. Thirdly, the reduction in the social pooling contribution rate has a positive economic effect; it not only promotes economic growth and social welfare, but also contributes to the multi-tier pension insurance system. Therefore, it provides the evidence for the reform of the reduction in the contribution rate of basic pension insurance system for urban residents in China.