The illumination distribution in water is an important subject for efficient application research of fishing lamp. The traditional numerical calculation method could not accurately calculate the illumination distribution of the underwater fish gathering lamp in the offshore water. A new computing model of underwater illumination of fishing lamp was developed based on Monte Carlo method by using spectrum and luminous intensity of lamps and the inherent optical properties of seawater. Firstly, the vertical marine section was set as the target surface and divided it to some grids; secondly, photons with specific wavelength and direction were generated by random number method, and then the attenuation coefficient of each photon was calculated by using chlorophyll concentration data, optical characteristics of pure seawater and its relationship with absorption and scattering coefficients; finally, all photons were emitted, and their transmission paths were tracked. The photons were recorded only if they fell into the target surface and the illuminance value of each grid was calculated according to the recorded position of photons. The calculation method was verified with flume test through collecting water samples. The ${A_{{\rm{ds}}}}$—area of closed region which enclosed by 0.1 lx contour line in marine section which $ds$ m away from underwater fishing lamp and the ${I_{{\rm{ds}}}}$—maximum illuminance value in marine section were used to characterize the spatial distribution of light field in water. In this paper, the effect of chlorophyll concentrations, scattering type in the marine water and the scattering asymmetry parameters on the illumination distribution of LED fishing lamp and MH fishing lamp were analyzed through numerical simulation. The results showed: (1) with the chlorophyll concentration increases from 1 mg/m³ to 5 mg/m³, the ${A_{5{\rm{m}}}}$ of LED lamp decreased from 1073.90 m² to 292.87 m², which means ${A_{5{\rm{m}}}}$ has reduced by 72.73%; the ${A_{5{\rm{m}}}}$ of MH lamp decreased from 1094.51 m² to 304.57 m², which means ${A_{5{\rm{m}}}}$ has reduced by 72.17%; the ${I_{5{\rm{m}}}}$ of MH decreased from 972.76 lx to 454.42 lx, which means the ${I_{5{\rm{m}}}}$ has reduced by 53.29%; the ${I_{5{\rm{m}}}}$ of LED decreased from 956.48 lx to 318.60 lx, which means the ${I_{5{\rm{m}}}}$has reduced by 66.69%.(2) when scattering type of the ocean water is Mie scattering and Rayleigh scattering, the ${I_{5{\rm{m}}}}$of MH is 956.48 lx and 318.60 lx, respectively; the ${I_{5{\rm{m}}}}$of LED is 1057.96 lx and 992.42 lx, respectively.(3) with the scattering asymmetry parameters increases from 0.80 to 0.99, the ${A_{5{\rm{m}}}}$ of MH lamp decreased from 1094.89 m² to 1336.40 m², ${A_{5{\rm{m}}}}$ increased 22.05%; the ${A_{5{\rm{m}}}}$ of LED lamp decreased from 1076.52 m² to 1325.21 m², ${A_{5{\rm{m}}}}$ increased 23.10%; the ${I_{5{\rm{m}}}}$ of MH increased from 962.52 lx to 1122.48 lx, ${I_{5{\rm{m}}}}$increased 16.62%; the ${I_{5{\rm{m}}}}$ of LED increased from 980.74 lx to 1123.22 lx, ${I_{5{\rm{m}}}}$increased 14.53%. It was found chlorophyll concentrations had a significant effect on illumination distribution of fishing lamp, while the scattering type and the scattering asymmetry parameters on the illumination distribution had no significant effects. The numerical simulation method proposed in this paper can accurately calculate the illumination distribution of fishing lamps in offshore waters, which would provide scientific basis for fishery supervision and reasonable application of fishing lamps.