The explosion of data in big data environment brings great challenges to the system structure and capacity of storage system. Nowadays, the development of storage systems tends to be large capacity, low-cost, and high performance. Meanwhile, storage devices such as conventional rotated magnetic Hard Disk Drive (HDD), Solid State Drive (SSD) and Non-Volatile Random Access Memory (NVRAM) have limitations caused by their intrinsic characteristics, leading to the fact that a single kind of storage device cannot meet the requirements above. Hybrid storage which utilized different storage medium was a good solution to this problem. SSD, as a kind of memory wiht high reliability, low energy consumption, high performance is more and more widely applied to hybrid storage system. By combining magnetic disk with the solid-state drives, the advantages of the high performance of SSD and the low-cost, high-capacity features of HDD were taken. The hybrid storage could provide users with large capacity of storage space, guarantee the system's high performance, at the same time reduced the cost. The current research status of the SSD-HDD hybrid storage system was described, different SSD-HDD hybrid storage systems were summarized and classified. In view of two different HDD-SSD hybrid storage architectures, the key technologies and insufficiencies of which were discussed. Prediction of trend and the research focus in the hybrid storage future were discussed at last.