The classic method to solve interaction between two-level atom and classic electric field is used to figure the several rates equation directly. The density matrix' method is used to establish the Hamiltonian operator of interaction between single two-level atom and classic electric field,and the differential equation is figured out,which is satisfied the density matrix of two-level atom system. The result shows that the density matrix non-opposite angle element never be zero at any time,and the quantum coherence characteristics of two-level atom system can keep well in this interaction model.