Underwater wet welding technology has been widely used in recent years, however, there is a lack of research on its mechanism. In this paper, the plasma temperature and electron density of the arc-inducing process of underwater wet welding are studied by means of spectral analysis. Firstly, the underwater wet welding arc spectrum diagnostic platform was built, and the current, voltage and spectral signals in the welding process were collected synchronously. The arc strike process is defined based on the analyzed current and voltage signal data. On this basis, the spectral signals of 5, 10, 15, 20 and 25 ms are collected by the time delay recording function of the spectrometer. The Fe element line and the H element line required to calculate the plasma temperature and ionization density are calibrated. In order to ensure the accuracy and reliability of the calculation results, five sets of data were selected at different times of the arcing, and the five sets of data were averaged. Five suitable spectral lines were selected in the calibrated Fe element line, and the underwater wet welding arc plasma temperature at different times of the arcing was calculated by Boltzmann diagram method. According to the H_α spectrum detected by the spectrometer, combined with the Stark line broadening theory of plasma emission spectroscopy, the electron number density at different moments of underwater wet welding arc is calculated. The calculation results show that the temperature changes of underwater wet welding arc plasma show different characteristics at different moments of arc ignition, In the arcing 5 and 20 ms respectively, the temperature values appear peak respectively, and the temperature value reaches 4 414 K at the last steady arc time. The electron number density also differs at different times of the arcing, and also peaks at 5 and 20 ms. The changing trend of arc plasma temperature and electron number density at different timings of arc ignition verified the formation of arc accompanied by the breakdown of space gap. The calculation results can further explore the physical nature of underwater wet welding arc-ignition process from the perspective of arc physics, and provide an important reference for guiding and seeking more effective arc-ignition methods.