The paper introduces a correcting system of an automobile speed-meter based on 89C51 microprocessor. By using coefficient multiplier, the correcting signals will be in the range of 10 to 9 999. 9 Hz with a resolution of 0. 1 Hz. The waveform of the correcting signal outputted from the system is coded with CPU and address modification circuits sharing the same memory. The system is characterized by high precision, good universality, and strong ant-i disturbance with the output waveform set at will.