The invention discloses a speed control system of an automobile. The speed control system comprises a first automobile door locking sensing device, a second automobile door locking sensing device, a third automobile door locking sensing device and a fourth automobile door locking sensing device which are respectively mounted on four automobile doors of the automobile and are respectively used for judging whether locks of the four automobile doors of the automobile are locked or not. The first automobile door locking sensing device, the second automobile door locking sensing device, the third automobile door locking sensing device and the fourth automobile door locking sensing device respectively output a first speed limiting signal, a second speed limiting signal, a third speed limiting signal and a fourth speed limiting signal to a control unit when sensing that the corresponding automobile doors are not locked; the control unit outputs a starting signal to an automobile speed limiting unit when receiving an optional one or more of the first speed limiting signal, the second speed limiting signal, the third speed limiting signal and the fourth speed limiting signal, so that the running speed of the automobile can be limited by the aid of the automobile speed limiting unit and cannot be higher than a preset automobile speed.