A planar point set X is called a k-distance set if there are exactly k distinct distances defined by every two points in X, and the longest distance is called diameter D. The set of the endpoints of all diameters is denoted by X_D . Let m=m(X)=|X_D| be the number of elements of X_D, and the diameter graph DG(X_D) be all diameters in X. There are many results on determining the value of g(k) when k≤6, where g(k) is the number of points of the largest point set having k distinct distances. We consider planar point sets for the case of k≥7. Firstly, we perform an analysis on the degree value d(v) of all vertices in k-distance DG(X_D) for m=|X_D|=2k-1, and obtain that d(v)≤2. Based on this result, we research the case of 7-distance. We get X_D=R15-3 when the 7-distance sets DG(X_D)=P10∪P₂. The result provides a theoretical foundation for further discussions on 7-distance sets.