Entanglement characteristics in a system of a two-level atom interacting with a number-phase state (NPS) field are investigated by using the fully quantum theory. The entanglement degree expression of the system is calculated. Influences of the initial atomic state, parameters of field phase, largest-photon number, parameters of light field and transition photon number on the system entanglement degree are analyzed. Results show that when the initial atomic state is in superposition state, and the field phase parameter is small relatively, the system entanglement degree is large relatively. When the appropriate largest-photon numbers and transitional photon numbers are chosen, the smaller the field parameter is, the more obvious the entanglement phenomenon will be.