A method for expanding lifetime of a sensor node in a wireless sensor network and a system therefor are provided to determine total sleeping time of each sensor in consideration of connectivity of a network, thereby expanding lifetime of a sensor while maintaining desired network reliability. A reliability value guaranteeing network connectivity is set according to connection significance of a network(430). Operation probability of the sensor node is calculated(440). If the operation probability is larger than the set reliability value, total sleeping time with the minimized operation probability is calculated(460). The operation probability of the sensor node is a value dividing a power amount of the sensor node by sum of the total sleeping time and the power amount of the sensor node. Connection significance of the network is transmission significance of data delivered in a network.