Abstract: To maintain shape similarity of features is one of evaluation criteria before or after in map generalization.The article derives shape similarity of line-feature from shape descriptor of polygon——rebuilds conjugate polygon of line-feature consisting of mirroring-part of original feature using the axis of symmetry through the first point and the end point of itself,gets a shape vector of the line feature,which is the shape descriptor of its conjugate polygon and further requires the similarity distance among line-features through Euclidean distance between two vectors.To ensure certain evaluation accuracy,the K value is determined by calculating Kth item X,Y value of Fourier series.In the end,the shape fidelity of several simplification methods to given curve is analyzed to provide the reference to select which algorithm will be used.The model is simple,with mathematical rigor,and highly practical.