Radiation distribution in a photoreactor with multiple mediums is solved by the P-1 model. The UV lamp is included in the computational domain. An additional source term method is presented to describe the generation of UV light, which can avoid the use of analytical lamp emission model at the lamp surface. The boundary condition for incident radiation at the semi-transparent wall between adjacent mediums has been derived to link the P-1 equations in adjacent mediums, which enables the present model to adapt to the photoreactor with complicated structure. The predicted incident radiation agrees well with the experimental data in literature. The effects of absorption coefficient, scattering coefficient and phase function of lamp plasma on the radiation distribution are discussed in detail.