Abstract: The weight window is one of the most widely used variance reduction techniques. When used judiciously, it can save substantial computer time. Because the MCNP program can not calculate the volume of some special cells, for example, repeated structure cells or asymmetrical cells, the adjoint weight window generator of MCNP is almost unavailable for a complex geometry model. A mesh-based adjoint weight window generator was provided in the paper. It utilized an extra virtual mesh model where original model is enclosed, and no modification was made on the original model. It can be used for almost all models. The new method was successfully used to a repeated structure model, and the FOM value greatly increased. A satisfactory simulation result of a photon skyshine experiment model was obtained by the new method. Compared with the experiment result, the relative difference of the new method is 1.2% only. The FOM of the new method is almost eight times as large as that of the direct Monte Carlo simulation.