A fast model is developed for the simulation of the mask diffraction spectrum in extreme ultraviolet lithography. It combines a modified thin mask model and an equivalent layer method and provides an analytical expression of the diffraction spectrum of the mask. Based on this model, we perform a theoretical analysis of the mask shadowing effect. Mathematical expressions for the best mask (object space) focus position and for the required correction of the mask pattern size are derived. When the mask focus is positioned in the equivalent plane of the multilayer, the amount of pattern shift is reduced. When the mask pattern size is corrected using the derived formula, taking a space pattern with a target critical dimension (CD) of 22 nm as an example, the imaging CD bias between different oriented features is below 0.3 nm.