According to the defect characteristics and the failure mechanisms, the defect detection technique of rail is discussed based on the magnetic flux leakage (MFL), and a test system of magnetic flux leakage based on AT89C51 is proposed. This paper firstly introduces the theory of magnetic flux leakage, and then combining with electron and circuit theory and MCU technology we want to develop a test system of magnetic flux leakage. This test system includes the processes such as signal acquisition, preprocessing, A/D conversion, results preserving, data serial communication and real-time analysis on PC and so on. The software design of MCU is based on modularization design, which is divided into the modules of data acquisition, the modules of data preserving and the modules of data serial communication.