An Internet of Things (IoT) network system and a networking method thereof are provided. The IoT network system is connected to a wireless access point and includes a plurality of network nodes and an idle node. The plurality of network nodes are communicatively connected to the wireless access point through a tree topology to form a first tree structure. In order to join the IoT network, when communication signal quality of the idle node with respect to the wireless access point is greater than a preset value, the idle node is communicatively connected to the wireless access point to form a root node of a second tree structure. When the communication signal quality of the idle node with respect to the wireless access point is not greater than the preset value, the idle node joins the IoT network by using one of network nodes in the first tree structure as a parent node.