A quantum theory for pulse-train from the mode-looked laser has been developed. In the theory the author uses "coherent superposition" model, i. e. the atomic states are coherent after absorbed photons from different modes of a mode-locked laser. So the final atomic state should be coherently added. The resolvent operator and projector operation are used to derive the operator U. Considering the phase relationship between the modes, the light field are described by multi-mode coherent state. This method can be used to describe the resonant multi-photon processes that the pulse train fxom the mode locked laser interacting with atoms.