We study the Knieper measures of the geodesic flows on non-compact rank 1 manifolds of non-positive curvature.We construct the Busemann density on the ideal boundary,and prove that if there is a Knieper measure on T1 M with finite total mass,then the Knieper measure is unique,up to a scalar multiple.Our result partially extends Paulin-Pollicott-Shapira's work on the uniqueness of finite Gibbs measure of geodesic flows on negatively curved non-compact manifolds to non-compact manifolds of non-positive curvature.