A hydrothermal carbonization reaction is performed in a reactor in a high-temperature high-pressure steam environment. As a material charging process of the reactor is performed in a batch manner, biomass should pass through a pulper in advance and be heated, and after the reaction is performed in the reactor, slurry outputting from the reactor should be cooled in a buffer tank. The invention provides a novel hydrothermal carbonization device to connect slurry cooling and biomass preheating. In specific, heat exchangers are disposed in the pulper and the buffer tank respectively, and are connected to form a heat cycling system, and therefore the slurry to be cooled can supply heat released by cooling to the biomass that should be heated in advance, thus obviously reducing energy loss in the system and allowing energy to be reutilized.