Abstract: As a high-efficiency, low-cost, and safe underwater cutting method, flux-cored wire arc cutting (FCAC) has broad application prospects. Due to the interference of the underwater complex environment, the stainless steel arc cutting mechanism of this method is still unclear. This research uses a combination of process tests, high-speed camera observation, and numerical simulation. Firstly, the relevant characteristic parameters of underwater cutting are determined through the process test; secondly, the semi-ellipsoid heat source is used to simulate the cutting heat source, and the movement and switching mode of the heat source is set according to the experimental observation; finally, the workpiece is meshed and ascertained the underwater boundary condition, the "birth and death" element method is applied to simulate the removal of molten metal during cutting; by comparing the simulated and actual temperature of measurement point and the kerf after cutting. The results showd that the kerf of underwater flux-arc cutting stainless steel is mainly divided into three types of shapes: "/ \" type, "| |" type and "\ /" type. The underwater stainless steel cutting process consists of multiple periodic continuous perforation processes observed by high-speed camera. It is also validated by simulation to predict the kerf forming process effectively, which helps to further optimize the process and control it effectively.