The fidelity in a system which is composed of a multi- photon transition moving two- level atom interacting with Polya state light is studied by means of the full quantum theory. The influences of the atomic initial state, field-mode structure parameter, the maximum photon number of the light, the optical field parameters and the values of the transitional photon number on the fidelity of the system are also analyzed .The results show that the velocity of atom effects the oscillation period of fidelity. The fidelity presents different patterns when transitional photon numbers are at some fixed values.