We report an experimental investigation of degenerate four-wave mixing spectrum of SO2. A novel beam-split technique is used to compensate the beam drift caused by the laser itself and disturbance from the environment. Degenerate four-wave mixing spectra of the transition (BB1)-B-1 <- X(1)A(11) for SO2 are obtained. The degenerate four-wave mixing signal intensity as a function of the total input laser energy is measured. The effects of the laser field intensity on the degenerate four-wave mixing spectrum are analyzed. Our results show that the degenerate four-wave mixing signal intensity rises with laser intensity and tends to saturate. Then, a dip begins to appear at resonance in the profile with increase in laser field. The existence of saturation for degenerate four-wave mixing signal intensity and spectrum profile confirms the prediction of previous theoretical work.