By considering lifetime uncertainty, this paper employs an OLG model within general equilibrium framework to analyze China’s urban public pension system. Using the condition for the steady-state of market economy to satisfy the social welfare maximization, we solve the optimal social pool benefit replacement rate. This optimal replacement rate depends on the population growth rate, survival probability in retirement period, capital share of income, individual discount rate and social discount rate. The simulations show that the optimal social pool benefit replacement rate rises with the life expectancy, whereas falls with the population growth rate. It should decrease when the life expectancy has risen and the population growth rate fallen because it is much more sensitive to the latter than the former.