To understand characteristic of the shapes of most tumors and the laser transmission mode as an internal light source during laser interstitial therapy, a double-layered media model in the shape of sphere with an interpolated optical fiber was established. Then, the modes of the photon transmitting in the tissue, at the boundary of the sphere, and on the surface of the inserted optical fiber were set, assuming the photons were launched at the center of the model. Finally, the photon migration in the model was simulated based on the Monte Carlo (MC) method in the Visual Studio (VS) program. The simulation results show that the optical fiber primarily affects the movements of photons that are moving near the surface from which they were launched. The energy absorption by the tissue under the photon-launching surface is greater than that in the above the surface. The absorption of the inner boundary increases with decreasing inner sphere radius. Compared with the traditional MC method, this model has more similarities with a real laser interstitial thermotherapy situation, which is of considerable practical significance for predicting the thermal damage range of laser interstitial thermotherapy.