The invention discloses an energy consumption managing system and method for a large-sized public building. The system comprises a wireless sensing network node, a WIA (Windows Image Acquisition) wireless gateway and a system server, wherein the wireless sensing network node is in signal connection with the WIA wireless gateway and the WIA wireless gateway is in network connection with the system server. System nodes are constructed into a network, a networked scale effect is achieved, and online monitoring can be performed on all energy consumption data and environmental information in the large-sized public building moreover, a large quantity of data can be analyzed and counted to form an effective control strategy and illumination and air-conditioning parameters can be optimized more effectively under the condition of ensuring comfort degree, so that optimal energy conservation is achieved.