The new method for isotope analysis based on degenerate four wave mixing (DFWM) is proposed for superfine structure researches and the quantum optics theoretical calculation is also introduced to get the percentage of isotopes. The self-oscillation frequency can be determined from the experimental phenomenon. Then, based on three order nonlinear polarizability, with quantum optics calculation of three ladder level system, the relationship of three order nonlinear polarizability and atomic density can be measured.