Simultaneous wireless information and power transfer(SWIPT) can improve the energy efficiency and resource reuse rate of sensor networks. However, current studies optimize the parameters of SWIPT to achieve system gains, ignoring the impact of channel quality changes on system energy efficiency. In order to solve this problem, this paper proposes a Sink node energy efficiency optimization strategy based on the optimal stopping theory for the SWIPT-enabled sensor-cloud system. First of all, the schedule sequence of downlink SWIPT and uplink information transmission is designed. In the downlink phase, the Sink node adopts the opportunity scheduling strategy and selects the time when the channel quality is good to start the downlink transmission. Sink node's energy efficiency is defined as the ratio of uplink throughput and downlink energy consumption realized by the system. Then, based on the optimal stopping theory, the problem of Sink node energy efficiency optimization is formulated and the existence of optimal stopping rules is proved. Finally, the optimal energy efficiency algorithm is designed to solve the optimal downlink SWIPT time of the system, so as to realize the optimal energy efficiency strategy of the sensor-cloud system. The effectiveness and performance of the optimal energy efficiency algorithm are verified by simulations, and the advantages of the optimal energy efficiency strategy are shown on system gains by comparing with other three strategies.