An active correlated infrared electro-optical detection alarm system is designed. It includes infrared detection and signal processing systems. Two correlated LH-A electro-optical switches with 3 m detection distance are used as intrusion detectors. When the modulated infrared light beam from the electro-optical switches is obstructed by an object, a switch signal will be produced. The signal is sent to AT89 micro-computer unit (MCU). After processing and judging, an alarm for passing objects at special directions is given out. And the alarm system can be used as a flow counter. The system has the advantages of high reliability, good stability and anti-interference.