Graph has been widely used to model the applications in social network, semantic web, computational biology and software analysis. Reachability query is one kind of basic queries in graph data. Currently, in allusion to graph, several index algorithms have been proposed to answer reachability query, however, they can not scale to large graph flexibly. To address the issue, a new index method called RIAIL (reachability index augmented by interval labeling) is developed in this paper. RIAIL labels each node with four-tuple. The first two elements are interval labels that encode reachability information of the spanning tree, and the last two elements encode reachability information of non-tree edges. RIAIL only needs to index when querying and the cost of index construction is little. Finally, a wide range of experiments on real and synthetic datasets are conducted to demonstrate that RIAIL can efficiently handle reachability query and easily scale to large graph.