The invention relates to a road automatic toll collection technology, in particular to an infrared vehicle separation signal detection method used for detecting of a road automatic toll collection system, and the device thereof. The invention adopts a single-send but multiple-receive scan round mode, and realizes decoding, and enlarging the received infrared signal of a detecting area through an infrared carrier frequency modulation and the scan round of an infrared beam, and regulates automatically a receiving gain; a control program performs region detection, judges the sizes of screening equipment in a detection region, and performs a detection, an alarm and a status display for a separation signal between two vehicles in the detection region. The invention has the advantages that the detection precision is higher, and the adaptivity is strong; the invention is applied to a road toll collection system, and the invention can reliably detect and separate a semitrailer, an independenttrailer and a rigid truck which pass.