The rapidly increasing renewable energy sources (RESs) in the power system bring a large number of disturbances due to the intermittent and random characteristics of power generation. At the same time, most of the RES units are composed of power electronic devices, which can reduce the system inertia and impact the stable operation of the power system. Therefore, more operating reserve resources are required to participate in the rapid regulation in the power system with a high proportion of RES. The rapid development of information and communication technologies make it easier and lower cost to control flexible loads. Based on the new network resources, for example, 5G, flexible loads can be regulated rapidly to participate in power system frequency control. In this paper, the models, categories, and functions of typical flexible loads are analyzed. Then, the research status of various flexible loads for participating in frequency regulation (FR) is combed in detail. On this basis, the advantages and disadvantages of various load control methods and their application scenarios are discussed. Finally, the development prospects of flexible loads for FR are proposed, so as to provide references for practical application.