Printing quality detection by digital camera is the direction of future development, but the problem of color space conversion accuracy need to be solved in the field of color detection. The color space conversion between RGB color space and CIEL*a*b* color space was achieved using polynomial regression based on subspace partition. RGB color space was divided into subspaces and polynomial models were created in subspaces with least square method. L*a*b* values of any RGB values can be obtained by solving polynomial equation according to their own space. The experiment showed that the conversion accuracy increases considerably by using this new method, which can satisfy the requirement of digital camera and lays the foundation for quality inspection of presswork.