Abstract: Aiming at the increasing requirements of image quality in traffic monitoring, a day-and-night optical system with large aperture and big target plane was designed. Through using the near-symmetric configuration, the design idea for adjustmenting the structural parameters of the first component and balancing day-and-night confocal plane was adopted to realize a large aperture and sensor optical system with excellent controlling of the vertical axis aberration and defocus amount. The multi-configuration optimization design was conducted in visible wavelengths and near-infrared wavelength of 0.79um. The day-and-night defocus amount of less than 0.005 mm could be achieved, the modulation transfer function (MTF) in full field of view is beyond 0.3 at the spatial frequency of 130 lp/mm in full field. Moreover,tolerance analysis and discussion for the optical system were done, and the deciding tolerance was suggested to be controlled for reducing the big deviation between- actual image and theoretical image resulted from manufacturing and producting.