The invention relates to an anti-cheating safety belt warning device with length detection function, comprising a vehicle speed detection module, a passenger detection module, a safety belt buckle state determining module, a warning module, and a safety belt length detection module that are connected serially to a main circuit; the safety belt length detection module comprises a safety belt length sensor, a fixed resistor and a safety belt length detection switch; a pull cord of the safety belt length sensor comprises two parts fixed to a safety belt and across a sensor spindle; a spiral wire duct, balls and a ball spring jointly form a slide rheostat, the slide rheostat and the fixed resistor are jointly and serially connected to a direct current circuit; the safety belt length detection switch and the safety belt buckle state determining module are connected in parallel to the main circuit, and the control end of the safety belt length detection switch is connected to the direct current circuit where the slide rheostat and the fixed resistor are located. The anti-cheating safety belt warning device with length detection function is high in reliability and detection accuracy, enables passengers and a driver to take slimmer chances, and provides increased wearing accuracy of the safety belt.