The cavity field spectrum of the Jaynes-Cummings model is investigated. The results for the initial fields in superposition state of zero and one photon Fork states (a qubit of cavity field) are presented. The relation of the spectral structure and the initial field qubit is analyzed. The result shows that the influence of probability amplitude of the initial field qubit on the spectral structure is sensitive with one-to-one correspondence between them. No information about the quantum phase of the qubit can be gained from the cavity field spectrum.