Aimed at antitheft alarm, the magnetic door alarm circuit system based on MCU is proposed. The system is easy to integrate, which can be used in the development of integrated circuit.The circuit system almost consist of magnetic door and alarm circuit, it also contains the program of MCU software. The basic requirement of alarming is related with sensitive reaction of circuit system. The paper firstly use Multisim to set up and simulate circuit to discuss and analyze alarm circuit based on relay and alarm circuit based on nand gate. The research proves that the ability of two circuits’ response is limited to millisecond level with the simulation of trigger pulse of MCU . It theoretically shows the correctness of the basic framework of the circuit system. At the same time, through the physical application test of the alarm circuit based on relay，it shows that the magnetic door alarm circuit system is sensitive and reliable.