In order to achieve uniform illumination of the mask surface, the converging lens of an immersion lithography illumination system was designed, and the error sources of the system telecentricity were analyzed. The uniformity deviation of the designed numerical aperture of the converging lens is within 0.2% , and the image telecentricity is less than 0.2 mrad . Taking the system’s telecentricity variation of 0.1 mrad , the numerical aperture uniformity variation of 0.1% , the point spread function variation of 20 μm and the focal length variation of 0.1mm as the tolerance basis values, the thickness tolerance of the converging lens is calculated to be ±0.02 mm to ± 0.05 mm and the surface inclination within 10" to 20". It is concluded that the design of the converging lens for the immersion lithographic illumination system is reasonable and the tolerance is practical to meet the non-uniformity lighting requirement on the wafer surface less than 3%.