Abstract: Survivability is an important issue in ATM. Following the description of the current ATM VP restoration mechanism, the survivability importance factor is introduced to describe the difference survivability requirement of various services in ATM network and the weighted restoration ratio is defined. To meet the survivability guaranteed requirement of critical services, a mechanism composed of two strategies, critical VC preassign protection connection and prior realtime restoration, is presented here. The result of analysis and simulation shows that this mechanism guarantees the survivability of the critical service and enhances the survivability of the network.