Underwater wet welding technology has been widely used in recent years, but there are few pieces of research on the physical nature of underwater wet welding arcing process now. In this paper, an underwater wet welding arc spectrum diagnosis platform was set up, voltage and spectrum signals in the welding process were collected simultaneously under different water depth conditions to define the arcing stage of underwater wet welding. The high-speed camera recorded the arcing process of underwater wet welding to observe the underwater dynamic changes such as arc and bubble more intuitively. On this basis, spectral signals of 5, 10, 15, 20 and 25 ms were collected by the spectrometer, the arc spectrum at a different time under different water depth conditions was obtained by changing the water depth condition. According to the principle of line selection, the Fe elements were selected as the characteristic elements to calculate the arc temperature of underwater wet welding. We selected five sets of data at a different time of arc initiation and averaged them by statistical analysis method to ensure the accuracy and reliability of the calculation results. Five suitable lines were selected from the Fe element line as the target line to calculate the arc temperature of underwater wet welding arc-initiating process. And the plasma temperature of underwater wet welding at different time under different water depth conditions was calculated by Boltzmann graphic method. The results show that the arc plasma temperature changes with the increase of arc initiation time, but its variation trend is not a simple linear increase, but a peak at different times of arc initiation. With the increase of water depth, the temperature of underwater wet welding arc plasma also increases, but the rising trend of arc temperature begins to change slowly. The increase in arc temperature at a depth of 40 m relative to a depth of 20 m is lower than the increase in arc temperature at a depth of 20 m relative to a depth of 0.3 m. With the increase of water depth, the increase of underwater environment pressure causes the arc to be further compressed, but the compression amount is limited. As the arc is compressed, the intensity of the arc light also increases. By means of spectral analysis, the physical nature of the arc-initiating process in wet-welding under water is learned from the perspective of arc physics, which provides an important reference for understanding the micro-breakdown mechanism during the arc establishment process and further improving the stability of the arc starting process in actual production.