In order to find a more accurate calculation method of relative illumination of lens image plane in a wide-angle lens system, the limitation of the existing calculation method is analyzed, and a method to calculate luminous flux by vector ray tracing based on aperture stop constraint mode is introduced. The expressions of feature points coordinates and illuminated area of the first optical surface from abaxial ray under the control of aperture stop are presented through vector analysis and derivation. According to the condition and the law of Lambert radiator, from object space of lens, two kinds of lens image illumination analysis modes under conjugated condition are set up. For similar imaging system and non-similar imaging system with large field, the expressions of image illumination and relative illumination are derived respectively. A basic scheme to increase relative illumination during optical designing is proposed.