Off-axis illumination (OAI) is one of the key technologies to enhance the resolution in extremely ultraviolet lithography (EUVL). For the optimizing of OAI specifications involving shadowing effect of mask, a novel model of OAI imaging is presented. It divides the incoherent incident light into a series of parallel light beams with numerical continuous directions, imaging the mask to the wafer plane with each of these beams based on Abbe imaging principle, and finally takes the superposition of the intensities to achieve OAI exposure in EUVL. Images at various defocus planes are equivalently achieved via adding extra defocus aberration into projection system. The model facilitates the simulation of the mask shadowing effect effectively. Then adapting the sidewall angle of the developed photoresist for criterion, the optimum specifications of OAI style are obtained for the exposure of 16 nm half-pitch dense line and space under the consideration of properties of photoresist and depth of focus requirement of the projection system with numerical aperture of 0.32.