In this article, simulations were conducted on a pilot-scale honeycomb monolith reactor for the photodegradation of formaldehyde. The monoliths in the reactor were treated as porous media, and the local light intensity across the porous media was modeled by the discrete ordinates model with one adjustable parameter, namely, the absorption coefficient of the monolith channel wall. This absorption coefficient was estimated by a trial-and-error method based on the observed modeling results of light intensity in a single channel. Then, the formaldehyde degradation in a monolith reactor was simulated by the proposed model. The proposed model achieved a reasonable agreement between the simulation results and the published experimental data. Based on the modeling approach provided in this article, the effect of humidity on the photodegradation of formaldehyde was investigated. The data indicate that the reaction is first-order in the reactants for the intended applications such as purification of air in office buildings and commercial aircraft. The optimal design of the monolith reactor was also investigated. The effects of a dimensionless geometric parameter and the number of lamps were investigated and correlated in terms of the formaldehyde conversion yield. Moreover, the optimal monolith-to-lamp-distance ratio, beta, was correlated as beta = 1.622n + 0.1038, where n is the number of lamps.