The invention provides a replacing device for battery boxes of an electric vehicle, which is used for taking down one undercharged battery box from the vehicle, conveying the undercharged battery box to a battery transfer rack or conveying one fully-charged battery box on the battery transfer rack, and mounting the fully-charged battery box on the vehicle. The battery boxes don't need to rotate when being replaced by the device, so that replacing time is saved, and the problem that a large rotating space is needed for replacing the battery boxes is solved. The device comprises a translational device, a mechanical arm and a lifting device, the mechanical arm has a bidirectional moving function, namely, the mechanical arm is capable of extending along the vehicle direction and extending along a battery rack, and two rows of rollers are arranged on the mechanical arm so as to be convenient for drawing off the battery boxes. The device has the advantages that the device consisting of various conventional products is lower in manufacture cost, small in size, flexible in action and fine in universality, the required moving passage width is obviously decreased, and occupied area for replacing is saved.