The invention provides a fluid UV sterilization system capable of automatically and stably keeping the fluid level, which includes a channel, a UV lamp module group, an electrical controller and a mounting bracket for UV lamp module(s), each UV lamp module comprising a frame and a plurality of UV lamps, the orientation of the frame and the lamps is perpendicular to or forms an angle with the general flow direction in the channel. A baffle is provided on either or both of the upstream and downstream of at least one of the UV lamp module(s), and the bottom of the baffle extends to such an extent towards the bottom of the channel that it at least reaches within the effective radiation distance of the UV lamp module group towards the fluid surface. The fluid UV sterilization system according to the present invention can control the fluid level automatically and stably, and can be used for the sterilization of domestic sewage, industrial sewage, recycled water, tap water and other kinds of water.