The characteristic of polarization chaos laser and coherence of semiconductor optical amplifier (SOA)-based fiber ring laser are investigated. By detecting the power and degree of polarization (DOP) output of the laser in laboratory, the process from amplified spontaneous emission (ASE) to stimulated emission of radiation, and to polarization chaos laser output is evolved. The coherence of chaotic light is verified employing Mach-Zehnder interferometer, and only the zero-order coherence is observed. Adjusting the optical-path difference of two arms, power output of the interferometer is measured, and visibility of interference streak is computed accordingly. The coherence time of 40 ps is got, which is basically consistent with the time of 43 ps computed by optical spectrum of chaotic laser. It is measured that the chaotic laser's coherence is independent of current of SOA. The superiority of chaotic laser's coherence in resolution and sensitivity is indicated when it is applied in the field of low coherence optical source detection.