With the optical Bloch equation and the density-matrix equations
the saturation effect of signal intensity has been studied for resonance degenerate four-wave mixing(DFWM)of two level system
and the signal intensity saturation behavior is obtained as a function of pump field intensity.The value of.the saturation pump intensity is determined by longitudinal relaxation time
T
1
and transverse relaxation time
T
2
.The theoretical result coincides with experimental result.