The annual production of waste cooking oil (WCO) in China exceeds 5 million tons, mainly composed of fatty acid glycerides. Through appropriate chemical conversion, second-generation diesel with fatty hydrocarbons as the main component can be prepared. Compared to traditional ester biodiesel (mainly composed of fatty acid methyl ester or fatty acid ethyl ester), green diesel has better performance and can be mixed with petrochemical diesel in any proportion. At present, research on the preparation of green diesel from WCO mainly includes hydrogenation catalytic process and hydrothermal catalytic process. This article compares the reaction mechanisms and parameters of the two types of processes, focusing on the research and development of reaction conditions such as catalyst and carrier selection, temperature, gas, etc., summarizing the applicable scope of each process, and providing a basis for selecting suitable processes. At the same time, prospects are proposed for the future development direction of the two processes: the hydrogenation catalytic process is relatively mature in commercial use, and the focus of future research is to improve the stability of the catalyst and the process economy; As an emerging technology that can produce in-situ hydrogen, hydrothermal catalytic technology should conduct in-depth research on eco-friendly and efficient hydrothermal catalysts and continuous hydrothermal reactions in the future, promoting the industrial application of hydrothermal catalytic technology.