The invention provides a device for cleaning a first mirror for a tokamak device by a direct-current cascade arc plasma torch. The device for cleaning the first mirror for the tokamak device by the direct-current cascade arc plasma torch comprises a cascade arc plasma torch generation system, a cooling water supplying system, a supporting system and a reflectivity detection system, wherein the cascade arc plasma torch generation system comprises a vacuum chamber, a cascade source, a direct-current power supply, a vacuum unit, a gas supply unit and a cooling unit; the cascade source is arranged at one end of the vacuum chamber; the direct-current power supply is electrically connected with the cascade source; the gas supply unit is connected with the cascade source through a pipeline; the vacuum unit is connected with the vacuum chamber; the cooling unit is arranged in the cascade source. The device for cleaning the first mirror for the tokamak device by the direct-current cascade arc plasma torch is simple, reasonable and compact in structure; the device is based on a direct-current cascade arc plasma torch technique; on the premise that an impurity is not introduced, the large-area and uniform cleaning of an impurity sedimentary formation on the surface of a sample of the first mirror can be realized; the impurity sedimentary formation is removed quickly.