A single expression based overall fitting method for scattered points of blade contours is proposed to address the large measurement errors of the existing tool measurement instruments when measuring severely worn tools based on machine vision and piecewise fitting. An algorithm of k-cosine curvature is used to extract the corners of the blade contour and the least square method is used for piecewise fitting. Based on this, a single expression of the blade contour used for object function fitting is determined and the nonlinear overall fitting is performed using Levenberg-Marquardt iteration. Finally, experimental results indicate that the overall fitting method can ensure the continuity of the fitting curve and improve the accuracy by more than 20% when measuring severely worn tools, drastically improving the measurement accuracy and robustness of the algorithm.