Off-axis illumination is an important application of resolution enhancement technology in projection lithography. An approach of designing freeform lens for off-axis illumination (OAI) in optical lithography is propose. Based on the mapping between the source and the target and conservation law of energy, a set of partial differential equations are obtained and numerically solved, and then optical performances of the freeform lenses are simulated. With the method, free form surfaces are designed to achieve an annular, dipole, quadrupole lighting. The shape and intensity of illumination distribution on the target surface can be controlled precisely. Also, the illumination efficiency is over 90% and good uniformity can be achieved.