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An on-demand routing protocol for mobile ad hoc networks is one that searches for and attempts to discover a route to some destination nodes only when a sending node originates a data packet addressed to those nodes. In order to avoid the need for such a route discovery to be performed before each data packet is sent, such a routing protocol must cache routes previously discovered. Using simulations, we found that the performance of the networks which utilized DSR protocol became to be worse with the reduction of the packet sending rate. This paper aims at that phenomenon, indicates it is because that DSR routing algorithm can not check and purge the invalid routes in time, and puts forward improved strategies of caching, searching and discarding routes in the route cache by using signal strength and time stamp thresholds. By using ns2 simulator, we studied network performance using improved routing algorithm and DSR, validated that the improved algorithm performed better than DSR obviously.