Energy-efficient and reliable beacon dissemination is essential for many protocols and applications in wireless sensor networks (WSNs). In low-duty-cycle sensor networks, a node may miss incoming beacons due to unsynchronized active/sleep modes as a result of clock errors. Therefore it is critical to guarantee beacons arrive at the receiver in the right time. In this paper, we analyze this problem of unsuccessful beacon reception, and present Guard Beacon, an optimal beacon scheduling strategy for energy efficient and reliable beacon transmission. By jointly investigations on the beacon reception probability and power consumption on beacon sending and receiving, we find an optimal iterative solution as well as a sub-optimal analytical solution, of how many beacons should be sent, and when to send. The strategy is implemented in a real-world testbed. The evaluation results show that the proposed Guard Beacon can not only reduce the power consumption on beacon dissemination compared with other strategies, but also ensure reliable beacon reception, in low-duty-cycle wireless sensor networks.