With the increasingly strict diesel standards in the countries of the world, deep processing of diesel oil to ultralow sulfur levels is receiving more and more attention. This paper mainly reviews the reaction mechanism, catalysts, and process conditions of diesel hydrodesulfurization, and it provides new research directions for producing ultralow sulfur diesel (ULSD). In terms of mechanism, the sterically hindered sulfides, nitrides, aromatic compounds, and hydrogen sulfide affect the direct desulfurization and hydrogenate pathways of the hydrodesulfurization reaction to varying degrees. In order to eliminate these effects, the properties of high-dispersion active metals, large pore size, high specific surface area, high content of medium-weak acid, and a certain amount of Bronsted acid support are beneficial to further improve the activity of the catalyst, to produce ULSD that meets market demand. This article also reviews the influences of process conditions (for instance, temperature, hydrogen pressure, liquid hourly space velocity, and hydrogen consumption) on the diesel ultradeep hydrodesulfurization reaction, and it finds those moderately high temperatures and high hydrogen partial pressures, as well as low space velocity, to be beneficial. In short, the development of new catalysts is the current research hotspot in the field of ultradeep hydrodesulfurization of diesel, and further research is still needed.