A momentum transfer of a moving three level atom interacting slantingly with a standing wave laser field is analysed. The results have shown that a stimulated Raman transition will be dominated, when some conditions are met by both the atom and the light field. If the light field seen by the atom is a “π/2 pulse” light, the atom will be in a coherent superposition state after interaction, the one state is the same as the initial state of the atom, and another state is a new state in which the degrees of atomic internal freedom and the center-of-mass momentum of the atom are all changed, especially, the momentum is changed by twice single-photon recoil momentum. Nevertheless, the probability of the atom in the two states is the same. If this mechanism is used for atomic wave interferometry, it is possible that atomic wave packets are coherently splitted.