Dynamic range is a very important figure of merit to an imaging system in astronomy since it decides the range of brightness we can observe. This paper describes the design of a CMOS camera with extended dynamic range in which the CMOS sensor achieves high dynamic range by its dual slope response. We first established a model of how the dual slope response works, and gave a method to restore the image from dual slope response to linear response with the extended dynamic range. Then the data needed in the restoration to linear image was obtained in the laboratory by experiments using stable light source based on the model. At last the results of high dynamic range linear response images are shown using these experiment data.