A routing algorithm for Ad Hoc Networks associating reactive and proactive protocols, called preemptive algorithm, was proposed. The network nodes estimated valid duration of the link when the received signal strength was below the dangerous power threshold. If the estimated valid time was less than the response time，finding an alternative local route was triggered and the to-be-broken route was substituted before the dangerous link was broken. The simulation result shows the extended AODV protocol with the preemptive algorithm proposed significantly reduces the number of lose packets due to the broken paths，decreases the packet delivery delay in the situation of heavy traffic.