Abstract: Monte Carlo simulation is an effective numerical simulation approach to calculate illuminance at target region by discretizing beam into photons and calculating illuminance based on tracking photon path. In order to calculate the horizontal illumination distribution of the fish lamp accurately, taking the metal halide fish lamp as an example, we analyzed and optimized the photon number and photon emission model in the process of Monte Carlo simulation. For the photon number, we carried out the numerical simulation by setting different photon numbers to analyze its influence on the stability of simulation results. Meanwhile, we presented a photon emission optimization model based on the luminosity distribution data of fish lamp and verified by the measured data. The results show that: 1) The larger the number of the simulated photon, the better the stability of the simulation result. Taking the calculation point with a horizontal distance of 5 m from the lamp as an example, when the value exceeded 10⁸, the coefficient variance (CV) was 0.07% and when the value exceeded 10¹⁰, the CV was 0.05%. 2) Byusing the optimized model, the mean relative error (MRE) between the simulated and measured data reduced from 0.051 2 to 0.045 3. In addition, we calculated the light field illumination of fishing boats in the actual working environment by using the optimized model, and the average error between the calculated value and measured value was about 0.052 3.