To establish an optimization method for UV systems, numerical models were developed based on UV intensity field, flow field and equivalent reduction dose (RED). The model was verified using bioassay on the condition of the UVT (80％-95％) and the flow rate (240-600 L／h) with an annular UV reactor. Results showed that the model predicted the disinfection efficiencies with good accuracy. RED from simulations and experiments under different conditions had a difference of 3％-12％. RED increased with the increase of flow rate and decrease of the transmittance. However, transmittance had merely a slight impact, about 3％ on RED of high flow (600 L／h). Visualization images showed that near the inlet of the reactor, the UV intensity was weak while the velocity was large adjacent to the wall, which had a negative effect on disinfection efficiency. This numerical model provided a basis for UV system optimization and dose synchronization.