Determination of light absorption distribution in biological tissue co ntributes to selecting strategy and optimizing dose for biomedical applications.As an efficient illumination scheme for cylinder-li ke hollow organs such as prostate,interstitial light delivery is commonly employed in the treatment method of photodynamic therapy.In this paper,a simplified optical model of internal light irradiated columnar tissue using cylindrical diffusing source at 633nm is deve loped.The light distribution in the tissue is predicted noninvasively by combining the optical model with Monte Carlo method. The related simulation results are almost consistent with the experimental results.The related simulation method and model have an important value for the study of light transport i n the complex tissue and would be helpful to the choice of dose and treatment plan when using an interstitial cylindrical diffusing source in clinical medical applications.