The invention discloses a safety start method of a tree topology in an IPv6 over low power wireless personal area network (6LoWPAN), belonging to the filed of networks. The method comprises mutual authentication and trust assessment. Before node deployment, a network deployer allocates a unique identification (ID) and key for each node, so that mutual authentication can be achieved between two nodes if only the ID of the opposite party is known; and when a child node selects father nodes to access a network, trust assessment is carried out on the candidate father nodes, and a utility value method in a multi-target objective decision technology is utilized to comprehensively assess three indexes, namely the number of hops from the father node to a base station, consumed energy and time delay, so as to select an optimal father node. According to the method, node energy is saved, mutual authentication can be carried out by only IDs of the nodes and a small amount of information exchange but without the participation of a base station, and attack of malicious node can be resisted. Due to the introduction of trust assessment, the sideness solely depending on energy or hops is avoided, the energy consumption of the whole network is balanced, and the life of the network is prolonged, thus the method can be applied to scenes with small amount of node movement.