Degenerate four wave mixing is treated using the quantum theory of fradiation. Starting from the energy conservation and momentum conservation laws and using the transition rate-formula for quantum states, the equation of motion for photon number densities of four beams is derived and exact solutions are obtained. With these solutions, change of nonlinear reflectivity with the pump waves and the probe wave, and saturation, and parameter oscillation arc-described. Besides, comparisons are made between our theory and classical theory as well as Bloom's large signal experiment. It is shown that our theory is more accurate and reasonable.