A high precision detection system consisting of displacement transducer DA-0.5, AT89S52 single-chip computer and ADS7841 DAC has been developed to solve the rapid online sheet thickness detection. On the condition of work progressing, the detection system's functional completeness is quite perfect, with the detected metallic gasket seal to verify it. The experiment results show that the system achieves stability and detection accuracy in compliance with requirements. Combined with the use of this system, the thickness measurement of sheet online with high precision and speed can be performed.