Natural microalgae with high ash content are common in water environment. Converting them into biofuels not only meets the energy demands but also improves the aquatic environment. This study aims to explore the physicochemical properties and molecular structural features of hydrochars derived from hydrothermal treatment of natural microalgae. Meanwhile, the combustion behavior and kinetics analysis of hydrochars were also evaluated. The hydrothermal treatment was performed with natural microalgae and its acid-washing microalgae under different temperatures from 260 to 340 degrees C to reveal the effect of ash on hydrochars properties. The results indicate that the ash significantly influences the functional groups composition and physicochemical property of hydrochars. The yields of hydrochars derived from deashing microalgae are lower than those of hydrochars derived from natural microalgae. However, the relative content of the C-C/C-H/C=C groups representing hydrocarbon carbon in hydrochars derived from deashing microalgae is higher than that of hydrochars derived from natural microalgae. Both natural microalgae and deashing microalgae contain the protein-N and pyrrole-N, and natural microalgae also contain a small amount of inorganic-N. The Brunauer-Emmett-Teller (BET) surface areas of hydrochars derived from natural microalgae and deashing microalgae are in the range of 5.97-10.29 and 21.34-34.74 m(2)g(-1), respectively. The thermogravimetric analysis results show that hydrochars derived from deashing microalgae have better fuel quality in view of the comprehensive combustibility indexes compared with hydrochars derived from natural microalgae, which is conducive to their application to solid fuels. The acid-washing pretreatment can effectively improve the utilization of natural microalgae.