A piecewise linear approximation based on vector slope is presented in this paper. Given a set of planar data points, generally, these points have sequence.The vector slope definition is introduced at first in this paper. The slope of every points in set G is computed. The points cluster in a group, where vector slope of these points are very near. Fitting every group to a line and linking these lines, a fitting curve can be attained. The vector slope defination has the message of size and direction, basic interval is (-4～4). The varing of vector slope in basic interval with angle(-180°～180°) is in one-to-one correspondence and the relation curve has good linearity. A piecewise linear approximation based on conventional slope has some problems that the slope value is infinite in some cases and the relation curve with angle has bad linearity. All these very effect the quality of fitting curve and confine the application range. This algorithm overcome these problems, so the authors can obtain a high quality fitting curve and the algorithm can be used in every graph. The time complexity is linear.