The invention discloses a water heater. The water heater comprises a shell, an inner container, a heating device, a water inlet pipe, a water outlet pipe and a sterilization device, wherein the inner container is arranged in the shell; the heating device is arranged in the inner container; the water inlet pipe and the water outlet pipe penetrate through the shell and the inner container and extend into the inner container; the sterilization device comprises a sterilization barrel, a driving part, an ultraviolet sterilization part and a brush; an opening is formed in the end part of the sterilization barrel; the sterilization barrel comprises a light-permeable structure covering the opening; the driving part and the ultraviolet sterilization part are arranged in the sterilization barrel; the brush is arranged at the outer part of the sterilization barrel and is in tight contact with the light-permeable structure; the driving part is used for driving the brush to rotate; the sterilization barrel is connected with the inner container; and ultraviolet light emitted by the ultraviolet sterilization part penetrates through the light-permeable structure and irradiates into the inner container. The invention further provides a sterilization method. According to the sterilization device of the water heater, the light-permeable structure of the sterilization device is cleaned and descaled through the arranged brush, and the sterilization device is guaranteed to have an efficient sterilization effect.