Wireless sensor network (WSN) has extended people’s ability of gathering information and integrated physical world with information world. In recent years, the emerging cloud computing has made remarkable development, which has injected new vitality into WSN. It leads to new applications and services, expands the ability of data processing and storage in WSN and even produces the sensor-cloud system. Sensor-cloud can deal with the information generated by underlying sensor network, and provide remote services for upper users, which enables users to gather, process, analyze, store and share sensed data according to their demands. In this paper, we investigate the existing sensor-cloud system in detail. We firstly introduce the background, system architecture and applications of the sensor-cloud, then summarize the characteristics of the existing sensor-cloud systems. We reveal the main problems of the existing sensor-cloud systems, such as poor bandwidth, high latency and high failure rate, et al. Moreover, we study the sensor-cloud structure based on fog computing, design the basic methods of solving the before-mentioned problems and discuss the future research directions.