Photoacoustic imaging has recently emerged as a promising imaging modality for prostate cancer. Knowledge of absorbed light distribution in prostate tissues is essential since the distribution characteristics of absorbed light energy will influence the imaging depth and range of photoacoustic imaging (PAI). In this paper, a tumor-embedded prostate optical model was established. Light absorption distribution patterns of the tissue model through trans-urethral laser illumination using cylindrical diffusing light (CDL) and spherical diffusing light (SDL) were studied based on the Molecular Optical Simulation Environment (MOSE). In addition, the influence of laser energy and absorption coefficient of tumor on the light absorption in tumor was demonstrated. The results show that laser illumination from urethral allows the prostate tissues to obtain more efficient light absorption. The light absorption distribution of tumors irradiated by CDL has a relatively uniform characteristic in a large range, with its value around the light source less than that of SDL. Laser energy and tumor absorption coefficient has linear effect on the light absorption of tumor, which is consistent with the Beer Law. The conclusions will be helpful to optimize the laser source and to improve the imaging depth in a photoacoustic imaging system.