The invention relates to the control field of an electric bicycle, and discloses a control method and device for prolonging the driving mileage of the electric bicycle. The method comprises the following steps that (1) a controller judges whether a slow-down mode is started or not; (2) if the slow-down mode is started, the speed at present is collected to be compared with a preset speed; (3) the current at present is collected to be compared with a preset current; (4) the duty ratio of PWM is adjusted. The control method for prolonging the driving mileage of the electric bicycle is used, when a rider feels that the dump energy can not be used for getting the destination, the controller is manually switched to be in the slow-down mode. Under the slow-down mode, the controller reduces the duty ratio of the PWM, so that the output current is reduced, the rotating speed of the motor is reduced, the speed of the electric bicycle is reduced, and the energy consumption per kilometer of the entire electric bicycle is limited within a reasonable range. Through experiment measurement, under the same condition, the driving mileage obtained by the scheme of the control method and device is improved by about 61% than the driving mileage of a conventional scheme, the energy consumption per kilometer is reduced by about 40%, and the driving mileage under the same electric quality is effectively improved.