1. A design method of freeform imaging system comprising: step (S 10 ), establishing a initial freeform imaging system comprising a first initial surface and a second initial surface spaced from each other, wherein a plurality light rays from an object are redirected to form a plurality of image points adjacent to the ideal image points by the first initial surface and the second initial surface; step (S 20 ), constructing a second surface by calculating and curve fitting a plurality of second data points through a plurality of feature rays based on a given object-image relationship, wherein the first initial surface is fixed, and the plurality of feature rays are substantially intersecting the image plane at the plurality of ideal image points; step (S 30 ), constructing a first surface by calculating and curve fitting a plurality of first data points based on the given object-image relationship and Fermat's principle, wherein the second surface is fixed, and the plurality of feature rays are substantially intersecting the image plane at the plurality of ideal image points; and step (S 40 ), substituting the first surface and the second surface for the first initial surface and second initial surface respectively, and repeating step (S 20 ) and step (S 30 ) until the plurality of feature rays are intersecting the image plane at the plurality of ideal image points.