A mathematical description of the differentiation and development from DP thymocytes to SP thymocytes in the thymus is explored. According to the theory of a combination of instructive and stochastic model, a mathematical model is established. The results demonstrate that as the sites (expressing MHCⅠ/peptide) of thymic stromal cells(TSCs) increase, the CD4⁺8⁺ DP thymocytes lessen but CD4^-8⁺ thymocytes increase. And as the avidity of DP thymocytes with sites of TSCs increases, more DP thymocytes differentiate into SP thymocytes. How the thymic stromal cells mediate the differentiation from DP thymocytes to SP thymocytes is described.