The invention discloses a method and a device for externally powering a mobile phone, and belongs to the technical field of communication. According to the method and the device disclosed by the invention, a direct-current power supply is generated by an externally connected direct-current power inverter to replace a waste mobile phone battery to externally power the mobile phone, so that the mobile phone can be used in a continued manner. A power supply device of the mobile phone comprises an input end, a voltage converting portion and an output end, wherein the input end is connected with a mobile phone charger; the voltage converting portion mainly converts a 5V voltage output by the charger to be a 3.8V voltage which is the same as voltage of the battery; the output end is connected to a battery contact point of the mobile phone through a micro-clamp or a hook so as to replace the battery to power the mobile phone. Generally, the mobile phone battery cannot be recharged for use while being close to the service life, however, other parts of the mobile phone are normal in function, in this case, by means of externally powering the mobile phone directly by using the method and the device for externally powering the mobile phone disclosed by the invention, abandon of the mobile phone because the mobile phone battery is old and useless is prevented. With use of the method and device for externally powering the mobile phone disclosed by the invention, the mobile phone can be changed to be an alarm clock, an MP3 player, an MP4 player and the like, thus being effectively used.