A large number of animal manures has been produced in China in recent years, due to the ever-increasing demand for livestock and poultry breeding. Improper disposal can pose potential environmental and health risks. Alternatively, hydrothermal carbonization has been one of the thermochemical technologies to convert the waste biomass into high value-added products without drying raw materials. Therefore, it is a highly promising potential to convert the animal manures into high value-added carbon-rich materials using hydrothermal carbonization, in order to prevent environmental pollution and pathogenic microbe spread from the random accumulation. This review aims to clarify the effects of temperature, liquid-solid ratio, reaction residence time, and additives on the yield and physicochemical properties of hydrochars, according to the main components of animal manures under hydrothermal carbonization. Some mechanisms were evaluated to determine the specific reaction pathways, including hydrolysis, dehydration, decarboxylation, aromatization, Mailard reaction, and polymerization. An interaction was also identified between various factors and reaction kinetics. Among them, the reaction temperature was the most critical factor affecting the reaction pathways and characteristics of products. A mathematical model was then established to describe the hydrothermal carbonization, further evaluating the application effect on laboratory and industrial scale. There was an increase in the temperature and residence time during hydrothermal carbonization, leading to the hydrochar with a higher carbon content and calorific value, but the yield decreased significantly. Furthermore, the volatile content of hydrochar decreased significantly after hydrothermal carbonization, which was related to the reduction of hydroxyl, and carboxyl functional groups. The heavy metals and phosphorus were often fixed in the hydrochars in more stable forms, whereas, nitrogen and potassium were transferred to the liquid products. Medium temperature (180-220 ℃) for a short time (0.5-2 h) was conducive to maintaining the nutrient elements in the hydrochar for the better capacity of soil water holding. Moreover, the hydrochar also varied in the surface functional groups and pore structure, thereby serving as the potential biofuel, adsorbent, soil amendment, and precursor of functional materials. More importantly, the addition of CaO was greatly contributed to the pH and yield of hydrochar in the process of hydrothermal carbonization, together with the improved pore structure and the number of surface functional groups. As such, the acid increased the calorific value, specific surface area, fixed carbon content, and H/C, O/C atomic ratio of hydrochar. Co-hydrothermal carbonization had avoided the weakness of hydrochar produced by a single feedstock with high ash and heavy metals content, which were possibly limited factors for the potential applications in energy materials and soil amendment. Consequently, the porous carbon materials with high specific surface area, excellent pore structure, and adjustable surface chemical properties can be widely expected to prepare in the activation and modification process with the livestock manure hydrochars as a precursor. Microwave-assisted hydrothermal carbonization of animal manures can also realize rapid heating to shorten the reaction time for the higher product yield. A further study needs to be focused on the economic feasibility and large-scale application. This finding can also provide a strong reference to promote the application potential of hydrothermal carbonization in the treatment of animal manures.