Covalent organic frameworks (COFs) are a new category of materials and developing fast in recent years. COFs present low density, controllable porosity, and high surface area. Based on these merits, COFs have great potential in various applications, such as gas separation and storage, energy storage, catalysis, and many others. In this review, we summarize the synthesis of COFs from the aspects of design principles and synthetic reactions. In particular, we categorize the synthetic reactions of also COFs into six categories and introduce the advantages of each type of reaction. Moreover, we utilize several examples to illustrate how to construct COFs by these synthetic methods. In the end, a future perspective on the development of new synthetic methods for COFs is briefly mentioned.