In this paper, we established a double-layered spherical model of tissues with an inserted optical fiber, considering that a finite-diameter photon beam was emitted from the optical fiber embedded in the tissue during the laser-induced interstitial thermotherapy (LITT). Firstly, based on the Monte Carlo method, we obtained the absorption of an infinitely narrow beam in the tissue. Then, we attained the light transmission equation of a finite-diameter photon beam through the convolution between the incident light intensity and the Green's function. Finally, taking a Gaussian beam and a flat-topped beam as examples, we compared the changes in the absorption of the finite-diameter photon beam with or without considering the inserted optical fiber. The results show that the absorption of tissues for the flat-topped beam is smaller than that for the Gaussian beam. Besides, the influence of the inserted optical fiber on the absorption of the flat-topped beam is smaller, whereas its influence on the absorption of the Gaussian beam near the photon emission center is larger. Therefore, the influence of the inserted optical fiber on the absorption of light energy of tissues should be considered in LITT using the Gaussian beams. The proposed model follows the actual situation of LITT, which is of great significance for accurately predicting the thermal damage range of LITT.