The present invention provides an automatic driving-orientated vehicle longitudinal positioning system and method using variable grid-based image feature detection. According to a forward target obtained from data outputted from a high-precision navigation system and the distance of the target calculated by a road in high-precision navigation, an ORB (Oriented Brief) feature extraction algorithm based on a variable grid region which carries scale information is adopted to search the specific forward target in a vehicle-mounted binocular vision system and output the distance from the vision system to the forward target; and according to the distance and the installation position of the binocular system in a vehicle, a vehicle trajectory in the high-precision navigation can be corrected, and therefore, vehicle longitudinal positioning accuracy in the automatic driving can be improved.