Laser gas nitriding technology can quickly generate a nitride layer on the surface of titanium alloy, improve the surface hardness and wear resistance of titanium alloy, and promote the application of titanium alloy. Fiber laser was used to nitride of Ti-6Al-4V alloy, the electrical conductivity of the spectral emission region was measured using the probe method to define whether a plasma was formed in the spectral emission region during the nitriding process. The emission spectra of nitriding process were collected using a spectrograph. The spectral emission region was photographed using a high-speed camera to study the effect of process parameters on spectral characteristcs, the temperature of spectral emission region the quantity of plasma. Experiments show that the spectral emission region can conduct electricity during the process of fiber laser gas nitride of Ti-6Al-4V alloy, which indicates that the metal vapor plasma was formed in the spectral emission region. This was completely different from the nitrogen plasma formed in the process of CO₂ laser gas nitriding of titanium alloy. The number of metal vapor plasma was significantly affected by the process parameters during the process of fiber laser nitriding of Ti-6Al-4V titanium alloy. The metal vapor plasma can be produced in the spectral emission region when the laser power is higher, the scanning speed is lower, the defocusing is low, and the nitrogen content is high. The emission spectrum of the nitriding process is composed of continuous spectrum and liner spectrum. The continuous spectrum is mainly generated by thermal radiation, and the intensity of the continuous spectrum can represent the temperature of the spectrum emission region. The linear spectrum is mainly generated by the extranuclear electron transition of the plasma region, and the intensity of the linear spectrum can represent the quantity of the plasma. In the nitriding process, with the increase of laser power or the decrease of scanning speed, the continuous spectrum and linear spectrum were enhanced, indicating that the temperature of the spectral emission region increases and the number of plasma increases. As the defocus increases, the continuous spectrum and linear spectrum show a complex trend of decreasing first, then increasing and decreasing at last, which indicates that the temperature of the spectral emission region decreases first, then increase and decrease at last, and the number of plasma decreasing first, the increasing and decrease at last. Added a small amount of argon gas, nitriding process can be significantly influenced, the continuous spectra and linear spectra weakened dramatically, with the further increase of argon content, the linear spectrum and continuous spectrum continue to weaken, which indicated that the addition of a small amount of argon to nitrogen reduces the temperature of the spectral emission region and the number of plasma. With the further increase of the amount of argon, the temperature of the spectral emission continues to decrease, and the number of plasma continues to decrease.