In extreme-ultraviolet (EUV) lithography with off-axis illumination, the mask shadowing effects occur such as the orientation dependent pattern shift and critical dimension (CD) variation. Based on a simplified mask diffraction model, a theoretical analysis of the mask shadowing effects is proposed. The calculation formulas for the best mask (object space) focal-plane position and the correct value of mask pattern size are derived and used to compensate the mask shadowing effects. When focal plane of the mask is positioned on the equivalent plane of the multilayer, the pattern shift amount is reduced. When the mask pattern size is corrected using the derived formula, taking lines with the target CD of 22 nm as an example, the imaging CD bias is below 0.3 nm. However, with the decrease of the target CD, the formula error increases. It is necessary to consider the compensation of energy loss within the whole pupil that is caused by the off-axis illumination.