Quantum entanglement of a two-mode coherent field interacting with a Λ-type three-level atom system is solved exactly without rotating wave approximation, and the influence of mean photon number and detuning on quantum entanglement evolution are discussed. Numerical results show that the quantum entanglement period increases with the increasing of mean photon number. The quantum entanglement maximum decreases gradually with increasing of detuning, and the time reaching the maximum increases gradually. When the detuning is relative large, the quantum entanglement has better periodicity in a relatively short period. The small zigzag shaped oscillation of quantum entanglement evolution curve due to virtual photon effect is gradually enhanced with the increasing of mean photon number and detuning.