Disclosed are a pupil reshaping optical system of a lithography machine and a generation method of an off-axis illumination mode. The pupil reshaping optical system comprises a mode generation unit, a rotary wave plate, a polarizing beam-splitting unit, a ring No.1 generation unit and a ring No.2 generation unit. The modes of single-ring illumination and double-ring illumination under continuous variation of pupil plane light intensity and inner and outer diameter can be generated by means of the selection on corresponding diffraction optical elements in the mode generation unit, and the proper adjustment.