Abstract: Aiming at the problem of solid deposition in Shenhua direct coal liquefaction process, a study on the properties of deposits in hydrogenation reactors was carried out, and the formation mechanism of the deposits was inferred. The results show that the volume-average particle sizes of deposits from both the first-generation and second-generation technologies of Shenhua direct coal liquefaction are 913μm? and ?581μm? respectively, both of which are larger than that of raw coal, which indicates that obvious growth and agglomeration processes have occurred for the deposits in the hydrogenation reactors. Both types of deposits are dominated by inorganic matter, with ash contents of 91.34% and 83.19% respectively, while the carbon contents are 6.98% and 11.61% respectively. The crystalline phases in the two deposits are mainly the catalyst Fe1-xS, calcite CaCO?, and a small amount of quartz SiO?. Compared with raw coal loaded with catalyst, the Fe element in both deposits shows obvious enrichment, while the elements of Ca, Si and Na are diluted to some extent, especially Ca and Si. The formation mechanisms of solid deposits from the first-generation and second-generation technologies of Shenhua direct coal liquefaction are similar. The main components of the deposits are Fe1-xS and CaCO?, while the secondary components include NaCl, SiO?, as well as associated minerals in coal containing elements such as Mg, Al or K, and organic matter. The deposits mostly exist in forms of particles with relatively uniform radial elemental composition distribution, core-shell structure, or even three-layer radial elemental composition distribution.