It is usually to use a kind of flooding to detect and construct route during routing discovery in mobile ad hoc networks for such reactive routing protocols as dynamic source routing (DSR). When the being used routing fails, DSR must initiate a routing maintenance. When there is no other route to the destination node in its routing cache, the source node has to initiate the routing discovery by using the method of flooding again. This paper proposed an algorithm that uses nodes' position information to improve the routing maintenance of DSR, which not only decreases the times of flooding much, but also improves the performance of DSR. Simulations show the improved algorithm increases the delivery rate of DSR by 40-50%. And it decreases the end-to-end delay an order of magnitude. Furthermore, the end-to-end delay varies little when the mobility of nodes increases.