Terahertz CMOS circuits are much proper for THz communication and 5G communications application because of its small-size and compatibility with large scale silicon technology. Starting from THz CMOS doublers circuits, the state of the art of CMOS frequency multiplier circuit structures are investigated. The features and structures of push-push, injection locking and mixing doublers are introduced in detail. Through the analysis and comparison of above frequency multipliers, the advantages and disadvantages of different frequency multipliers in practical applications are summarized, which would lay a foundation for the small-size THz RF front-end in the future.