The quantum entanglement of the two mode coherent field interacting with a cascade three-level atom was calculated accurately with non-rotating wave approximation by using the complete quantum theory. The influences of the superposition between different atomic energy level at the initial time and the mean photon number on quantum entanglement evolution were considered. The results obtained by the numerical method show that the period of the quantum entanglement evolution becomes longer with the increase of the mean photon number when the atom initially at the single energy level, the quantum entanglement at the first few periods is reduced notably due to the fact of the atom is initially in the superposition state, it takes longer time for the entanglement to reach the maximum value with the increase of the mean photon number, moreover, because of the atom is initially in the superposition state, the periodic disappears. No matter which atom state, the quick oscillation due to the virtual photon process increases with the increasing of the mean photon number.