A simplified model is built to calculate the mask diffraction field in extreme-ultraviolet lithography. In this model, the analytical expression of the diffraction fields is derived by incident-ray tracing. The mask in the simplified model includes two parts, the multilayer structure and the absorber layer structure. The diffraction of multilayer is approximated as mirror reflection, and the diffraction field of the absorber layer is calculated using the modified thin mask model, where the absorber layer is equivalent to a thin mask located on a certain plane. The boundary pulse represents the boundary diffraction-wave effects. The geometric-optical wave through the absorber layer is modified by determining the position of the thin mask plane and the amplitude and phase of the boundary pulse. This modified thin mask model for the absorber layer can be used for 11 nm line/space patterns with oblique incident angles no more than 12°. Taking 22 nm dense lines pattern at 6° incident angle as an example, the results of the simplified extreme-ultravilet lithography mask model are consistent with the rigorous simulation.