In the present study, the photon absorption in a suspended photocatalytic reactor was simulated by adopting Monte Carlo method and the six-flux radiation absorption-scattering model. Different photocatalyst concentrations, i.e. uniform concentrations, linearly increasing concentration gradients and linearly decreasing concentration gradients, were taken into account. Simulation results indicated that increasing the photocatalyst concentration would cause higher photon absorption in the front layer but faster attenuation along the photon transfer direction. To the dilute solutions, the top layer photon loss was linearly influenced by the photocatalyst concentration. And the increasing concentration gradient resulted in higher photon absorption than the decreasing concentration gradient. The present work is expected to be of effective value for obtaining the optimal operating parameters for industrial photocatalytic applications. (C) 2014 Elsevier Ltd. All rights reserved.