Abstract: In this paper a full density matrix treatment is adopted to deal with the interaction between long-pulse, circularly-polarized light and the sodium atoms. The time evolution curve of each of the 24 sub-state’s population probability can be obtained by solving the Bloch equation of twenty-four hyperfine levels. It is found that during the long pulse time(more than 100 ns), the final transition is only between the sub-level 3S1/2(2,2) and 3P/3/2 and the probability of the excited sub-state has become steady. Comparison between the 2-level model and the full density matrix method proves the rationality of rate equation and the simplicity of 2-level model for the long pulse, circularly-polarized light.