The purpose of this paper is to estimate the mobility probabilities of income states between different age-sex groups. Since social security system usually implies the re-distribution of social wealth from the rich to the poor, the premiums and claims are usually determined by the insured persons' income states. Therefore, the mobility probabilities that describe the mobility of income states between different age-sex groups would be an important element for a simulation model of social security system. The input data of this study is the distribution weights of each income states of different age-sex groups. The approach attempts to simulate the mobility probabilities that could minimize the accumulated estimated errors of distribution weights in each age-sex group, in terms of a 3-Bands Model assumption. Our empirical results demonstrate that this model indeed fits on the actual Taiwanese data.