Nonlinear optical materials play a very important role in some advanced technology areas such as laser, optical communication, optical processing, high-frequency electro-optic devices and terahertz (THz) areas. Compared with the traditional inorganic materials, organic second-order nonlinear optical materials have the advantages of high electro-optic coefficient, fast response as well as easy processing. Recently, the research focusing on organic nonlinear optical materials and devices has been a hot area, especially the high-frequency electro-optic modulators and THz devices have received extensive attention. This review systematically describes the research progress in recent years and future development directions in the field of organic second-order nonlinear optical materials. The electro-optic properties and orientation stability of materials can be effectively turned through molecular design strategies and chemical synthesis, structural relationship between chromophores and polymers and doping techniques. However, there are still many challenges in developing organic nonlinear optical materials with excellent properties to meet the requirements of practical applications.