The fidelity in a system composed of a moving two-level atom interacting with an intermediate number-phase state was studied by means of the fully quantum theory. The influences of the atomic initial state, the largest number of photon, the parameter of light field, atomic motion, the parameter of field mode and the values of the transitional photon number on the fidelity of the system were analyzed. The result shows that the values of the average fidelity of system and light field decrease via increasing the largest number of photon or reducing the parameter of field. The larger atomic motion or field-mode structure parameter becomes, the greater values of the fidelity of system and light field are, and their oscillating frequencies are faster. As the transitional photon number is at different value, the evolutions for the fidelity of system and light field show periodic or random oscillation behavior. When atom is initially in a superposition state, the fidelity of system and field reaches its maximum, and the oscillating forms of the fidelity of system and field keep at a same manner.