The annual output of waste cooking oil (WCO) in China exceeds 5 million tons, and its main component is fatty acid glycerides. Through appropriate chemical conversion, it can be prepared into second-generation biodiesel with fatty hydrocarbons as the main component. Compared with traditional biodiesel (main components: fatty acid methyl esters or fatty acid ethyl esters), second-generation biodiesel can be blended with petrochemical diesel in any ratio for better combustion performance. Currently, research on preparing second-generation biodiesel from WCO mainly includes the hydrogenation catalytic process and the hydrothermal catalytic process. This article compares the reaction mechanisms and reaction parameters of the two processes, and focuses on the research and development of catalyst and carrier selection, temperature, gas and other reaction conditions. It summarizes the applicable scope of each process and provides a basis for selecting suitable processes. At the same time, it also looks forward to the future development direction of the two processes: hydrogenation catalytic process is relatively mature in commercial use, and the focus of future research is to improve the stability of catalysts and process economics; hydrothermal catalytic process, as an emerging technology capable of in-situ hydrogen production, should focus on in-depth research on green and efficient hydrothermal catalysts and continuous flow hydrothermal reactions in the future, so as to promote the industrial application of hydrothermal catalytic process.