Reachability query is a fundamental problem and has been well studied on static graphs. However, in the real world, the graphs are not static but always evolving over time. In this paper, we study the problem of historical reachability query on evolving graphs. We propose a novel index, named HR-Index, which integrates complete and correct historical reachability information of the evolving graph. A historical reachability query on an evolving graph can be converted into a static reachability query on its HR-Index and thus query efficiency can be improved significantly. We also propose two optimization techniques to reduce the size of HR-Index effectively. We confirm the effectiveness and efficiency of our method through conducting extensive experiments on real-life datasets. Experimental results show both vertex and edge size of HR-Index are far smaller than that of the evolving graphs and our method has at least an order of magnitude improvement in time and space efficiency compared to the state-of-the-art method.