Data center is not only an important IT infrastructure, but also a key support for enterprise Internet application. However, the resource utilization of data center is pretty low (only 10%~20%), which leads to a large amount of waste of resources, brings a huge extra operation and maintenance cost, and becomes a key problem restricting enterprises to improve the computing efficiency. By colocating online services and offline tasks, colocation can effectively improve the resource utilization rate of data center, which has become a research hotspot in academia and industry. This paper analyzes the characteristics of online services and offline tasks, and discusses the technical challenges faced by the performance interference between services and jobs. This paper summarizes the key technologies from the aspects of performance interference model, job scheduling, resource isolation and dynamic resource allocation, and discusses the application and effect of colocation systems in the industry with four typical colocation system. At the end of this paper, the future research direction is presented.