Abstract: The soil on Earth is facing serious threats such as soil erosion, loss of soil organic carbon, nutrient imbalance, acidification, and compaction. It is of great strategic significance to implement effective soil improvement measures for maintaining soil quality and promoting sustainable agricultural development. This study reviewed the potential and technical challenges of hydrochar (a kind of biomass charcoal) in soil ameliaration. Biomass charcoal is an excellent soil amendment with high carbon content, large specific surface area, high porosity, and strong adsorption capacity, which can improve soil properties through various physical, chemical, and biological methods. Biomass charcoal can be divided into biochar and hydrochar based on the preparation methods. Compared to biochars, hydrochars have milder preparation conditions, lower energy consumption, higher carbon fixation efficiency, and higher carbonization yield, which had attracted more attention from researchers. Hydrothermal reactions are usually carried out in a closed hydrothermal reactor. By setting certain temperature and pressure conditions, the material undergoes a series of chemical reactions such as hydrolysis, deamination, decarboxylation, polymerization, and aromatization, rapidly transforming into solid, liquid, and gas products. The solid product is usually referred to as "hydrochar". The composition and characteristics of hydrochar vary with different raw materials and preparation conditions. At present, many researchers at home and abroad have converted crop residues, sewage sludge, algae residues, and livestock manure into hydrochar through hydrothermal carbonization treatment, and conducted extensive researches to explore their potential applications as soil amendments. Numerous literature studies have shown that the application of hydrochar significantly affects the physical and chemical properties of soil (soil density, particle structure, pH value, electrical conductivity, cation exchange capacity, nutrient content, etc.), enzyme activity (soil sucrase, dehydrogenase, urease activity, etc.), microbial community (richness and diversity of bacteria and fungi), and soil functional genes (C, N, P and other element cycling functional genes). But the improvement effect of different types of hydrochar on different types of soil varie. The application of hydrochar to improve barren soils such as saline alkali soil, sandy soil, red soil, and acidic soil has been proven to be effective, demonstrating its enormous potential in soil improvement. However, the high phytotoxicity and low specific surface area of native hydrothermal carbon limit its application in soil environments to some extent. In order to address the deficiencies of hydrochar, researchers have made numerous attempts in the detoxification and specific surface area expansion of hydrochar. Techniques such as washing, thermal treatment, and adding chemical agents in physicochemical methods, as well as composting and aging in biological methods, have been employed to detoxify hydrochar, successfully achieving higher seed germination rates and positive plant growth effects. However, these studies primarily focused on improving the apparent physiological and biochemical indicators of plants without delving into the specific mechanisms of various detoxification methods. Moreover, methods such as acid modification, alkali modification, and metal salt modification were also used to dope elements and graft functional groups onto the surface of hydrochar, resulting in increased specific surface area and adsorption capacity, which is beneficial for the absorption and storage of various nutrients. In addition, there are no specific standard limits on heavy metal content in hydrochar established internationally, yet the level of heavy metals in hydrochar is a limiting factor affecting its comprehensive utilization. In the future, it is urgent to develop relevant technical specifications and standards while establishing a comprehensive monitoring and evaluation system to ensure its safety and effectiveness, thereby promoting the scientific and rational application of hydrochar. This study provides valuable information for further research and practical applications of hydrochar in soil environments, thus driving the continuous innovation and development of this emerging technology.