Most of the sulfur and nitrogen compounds in the feedstock are converted into H2S and NH3 during hydrocracking. During the reaction, part of H2S and NH3 are dissolved in the oil phase, and the other part is sometimes discharged to the outside of the unit through tail gas. Part of H2S reacts with ammonia in the stream to generate (NH4)2S and NH4HS, which are discharged after being washed with water. The existence of H2S has both advantages and disadvantages. Because most hydrocracking processes use non-noble metal catalysts, it is necessary to maintain a certain partial pressure of H2S in the system to avoid the reduction of sulfide catalysts. Too high partial pressure of H2S has no obvious effect on the hydrodenitrogenation activity and cracking activity of sulfurized hydrocracking catalyst, but it obviously inhibits the hydrodesulfurization activity and aromatic saturation capacity of the catalyst, especially when the partial pressure of H2S is too high, the noble metal catalyst will sulfide, resulting in the decline of activity. After hydrocracking products leave the cracking bed, a very small amount of olefins will react with H2S to produce mercaptan, which will aggravate the corrosion of products. If the sulfur content of raw materials is too high, it will not only form NH4HS, block the system, but also increase the corrosion rate of equipment. Generally, H2S in the system reaches more than 2%, so desulfurization measures must be taken to remove H2S in the high-pressure system. When the concentration of H2S in circulating hydrogen is too low, the metal components of the catalyst will decrease, thus reducing the hydrogenation activity of the catalyst and accelerating the deactivation of the catalyst. When processing low sulfur and high nitrogen VGO, the sulfur content in circulating hydrogen will be too low. China encountered this situation when processing Daqing and Liaohe oils, and the concentration of H2S in circulating hydrogen was sometimes only 200ppm or lower. Due to the above situation, the hydrodesulfurization activity decreases and the catalyst deactivation speed increases. When the concentration of H2S in circulating hydrogen is lower than 300ppm, measures should be taken to supplement sulfur in raw oil to keep the concentration of H2S at 300-500ppm. There are two ways to supplement sulfur for low sulfur raw materials. One is to add some high sulfur VGO；; If possible; Another method is to directly process low-sulfur oil by adding sulfides such as CS2, elemental sulfur, thioether or dimethyl disulfide into raw materials.