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\section{Introduction}  Edge detection is required for interpretation of gravity field data,and has been widely used in tectonic studies and mineral explorations(Paterson and Reeves, 1985). explorations\cite{Paterson_1985}.  To gain a better understanding of gravity sources, a variety of methods based on the use of directional derivatives of different orders have been developed to locate anomalous source boundaries.\\ In this paper,   we propose an edge detection method based on the analysis of eigenvalues of GGT. Considering the actual geological situation, the positive and negative density bodies usually coexist simultaneously. The proposed method has designed to outline the positive/negative density bodies with the corresponding values.