STL model has become a standard of RP technology, but the slice error due to tessellation is so high. To improve the precision of slice contours, a novel slicing method is presented to compute accurate contours curve based on STL model. At first, normal vector of two vertices sharing an edge of a triangle facet to be sliced is calculated, An interpolation curve is constructed in normal section to proximate the original curve in normal section in terms of the position of the two vertices and the tangent vector at the two vertices. The exact points belonged to slice contour are obtained by calculating intersection points between the interpolation curve and slice plane. At last, the accurate smooth contours are achieved by fitting the intersection points with cubic B-spline curve. Experiment shows that the presented method is feasible and reliable, the precision of cross contours from proposed slicing method approximates to direct slicing of original CAD model.