The application discloses a long-acting hydrogen-releasing microsphere in the stomach and use of the microsphere as a drug in treatment of gastritis, and belongs to the field of pharmaceutical preparations and hydrogen treatment. The composition provided by the application is composed of a hydrogen production micro-particle as a core and a sustained-release agent composed of hydroxypropyl methyl cellulose and ethyl cellulose. By selecting specific components and processes, the microsphere has good floatability and long-acting hydrogen-releasing capacity. The microsphere can float in the gastric juice environment and gradually release hydrogen, so that the sustained anti-oxidation and anti-inflammatory effects are realized, and the microsphere is suitable for treatment of gastric inflammation.