Based on discussing the reasonable target replacement ratio of the basic pension insurance, this paper evaluates the security level of Rural-urban Residents Social Pension Insurance by constructing actuarial model. The results show that the replacement rate is too small to meet the needs of basic pension insurance for rural and urban residents. In simulation analysis, the key point of improving the replacement rate of basic pension insurance is to ensure the growth rate of government subsidies on basic pension and the personal contribution ratio is not less than the growth rate of income of urban and rural residents. Increasing the investment return rate of the fund of individual account, extending the minimum payment years and raising pensioners' age are feasible measures to improve the replacement rate of basic pension insurance.