Abstract: A condition monitoring and fault diagnosing device for harvester hydraulic system was developed. Through monitoring the dynamic pressure, temperature and rotational speed of hydraulic pump of hydraulics on line, the combine operator and serviceman can exactly master the technical state of hydraulics. This device was designed innovatively, installed conveniently, operated simply and not only economical but also practical. It was proved by practical use that the function of this device was credible and stable. The device can be operated continuously in adverse circumstances. This paper presented the design and the realization means of the device.