A device and method for reminding following distance are provided. The following distance reminding device (FDRD) adapted for a first vehicle is capable of computing a following distance between the first vehicle and a second vehicle. The FDRD comprises an image acquiring module, a reminding module, and a processing module. The image acquiring module receives a vehicle image of the second vehicle. The processing module extracts a license plate image from the vehicle image, and computes the following distance between the first vehicle and the second vehicle according to image characteristics of the license plate image. The processing module computes a safe distance according to a driving speed of the first vehicle and the image characteristics, and determines whether the following distance is more than the safe distance or not, in order to control the reminding module to correspondingly send a reminding message.