The SI600S (4ktimes4k) CCD of CNEOST (China Near Earth Object Survey Telescope) now has been upgraded to STA1600LN (10ktimes10k), with the image region size that becomes larger, and the observation field of view (FoV) of telescope that enlarges from 4 to 9 deg^2. And correspondingly, the diameter of the original available FoV is expanded, from 3.14$^\circ$ to 4.28$^\circ$, which is improved by 36% compared with the original design. Moreover, the thickness of the field lens increases by 8.75mm. These two factors lead to the result that the off-axis aberration increases and image quality becomes worse. In order to solve this problem, based on the original optical design parameter of the telescope, we try to improve the image quality with the ZEMAX optical design software. The method is to design an additional field corrector to correct the off-axis aberration. The corrector is composed of two lenses with all spherical surfaces, and placed in front of the existing field lens. Furthermore, we put forward a new optical design which expands the available FoV from 14.38 to 28.27 deg^2.