From the view of the software fault-tolerance, in the distributed computing environment based on the network, for the property of the military command system and bank management system and in order to satisfy the reliability requirement of these applications, based on the group communication technology and by using the process replication technology, in this paper, a nested dynamic fault-tolerant model and a dynamic fault-tolerant algorithm are proposed. When the primary service process crashes, this algorithm can dynamiclly select one of backup process as a new primary service process and assure the state of all backup service processes is consistent.