A model based on the equivalent layer method is developed to simulate defective mask multilayer in extreme ultraviolet (EUV) lithography. In this model, the defective multilayer is divided into defect free region and defective region. The reflection coefficients of two regions in different locations are computed by the equivalent layer method. The spectrum of defective multilayer can be obtained fast and accurately. Simulation time of the proposed model is 1/9 times that of the waveguide method for a multilayer of 200 nm size. Compared with advanced single surface approximation (SSA) model and the simplified model based on SSA, the simulation accuracy of the multilayer spectrum and aerial image of the proposed model is improved. The errors of simulated amplitude and aerial image are also with little fluctuation in different defect sizes and incidence angles. Taking + 1 order diffraction as an example, compared with advanced SSA model and the simplified model based on SSA, in 6° incidence angle, the simulated amplitude error of the proposed model decreased as much as 77% and 63%, respectively.