A method and a system for network synchronization are provided. In this method, when a node attempts to join a network, a parent node is searched from all other nodes already joined the network per specific time interval. If the parent node can be found, sync-information sent by the parent node is received by the node so as to synchronize with the parent node and enter a working mode. When a sleeping instruction sent by the parent node is received, or a timer determines that a network ending time is reached according to the sync-information, the node switches to a sleeping mode. However, when the timer determines that a network wakeup time is reached according to the sync-information, the node attempts to join the network again. All nodes in the network can work and sleep simultaneously so as to extend the lifetime of the battery.