An improved method of distinguishing chaotic signal based on Wigner distribution (WD) is presented. The narrow band characteristic and the peak value fluctuation of the three-dimensional time-frequency spectrum are used to distinguish chaotic signal. The method is proved to be feasible by the calculation of Lorentz system. The WD of the chaotic signal is compared with that of the Gaussian noise, and the result indicated that their spectra can be distinguished. The WD is used to dispose the output light of the fiber ring laser base on semiconductor optical amplifier (SOA) too, and the result is similar to that of Lorentz system. So it could be verified that the experiment output is chaotic light. Three-dimensional WD accurately shows the spectral characteristic of the signal and can be used to differentiate whether the system is in nonstable state or chaotic state or noise. It is an intuitive and effective method for distinguishing the chaos either generated from dynamic equations or experiment output.