The utility model relates to the technical field of ultraviolet detection. The utility model relates to a switch cabinet, in particular to a switch cabinet arc online detection device based on ultraviolet detection. The device comprises a water containing device, a light source fixing device and an angle adjusting mechanism, the beneficial effects are that the light source pen is placed in the circular hole in the middle of the fixed circular ring; rotary hand rocker, hand rocker drives rotating shaft to rotate, rotating shaft drives helical gear to rotate, the helical gear is in meshing transmission with the lower surface of the rotating circular ring through helical teeth; the rotating helical gear drives the rotating circular ring to rotate; the four sliding blocks are connected to theupper surface of the rotating circular ring through threads respectively, the rotating circular ring drives the four sliding blocks to move inwards or outwards in the four sliding block sliding grooves respectively at the same time, the light source pen can be fixed inwards at the same time, and after the light source pen is fixed, emitted light can irradiate a water pool in the water storage tank.