To improve the accuracy of on-orbit spacecraft monitoring, this article proposes a method based on functional expert knowledge for on-orbit spacecraft state monitoring. The method first extracts the physical model of key parameters, and obtains the pipeline style and dynamic fine monitoring threshold through first year of on-orbit telemetry. Taking the Hydrogen-nickel Battery and Solar Array Driver Assembly key parameters of a geostationary satellite power supply subsystem as examples, the functional state monitoring method is studied and verified. The verification results show that the dynamic pipeline threshold based on expert knowledge can significantly improve the monitoring accuracy, which is one order of magnitude higher than the traditional fixed threshold monitoring accuracy.