Better distribution of the light energy in the field of view is the aim of the structure design of flashlight. Instead of the geometrical optics approaches, the Monte Carlo photon tracing method was used to analyze the structure design of flashlight. After analyzing and modeling of the camera flashlight structure (such as the light source, reflectors and Fresnel lens), and introducing cosine random number and other methods, we built up the model of a flashlight. At first, a single photon was traced using Monte Carlo method to simulate the behavior of the flights. Then, a large sum of photons were generated to simulate the real situation happened in the flashlight. At last, the light intensity and direction vector of photon distribution in the orthogonal coordinates was achieved. The simulated result meets the requisition and is very close to the measured one. The feasibility of using the Monte Carlo photon tracing method in the design of flashlight is proved.