The invention provides a steam turbine servo mechanism fault online detection and early warning method and system. According to the steam turbine servo mechanism fault online detection and early warning method, after it is judged that the noise elimination of time sequence data in a period is completed on the basis of a group of complete alarms, a decision tree is adopted to classify the time sequence data; fault feature waveform data mining is completed; the faults of a valve can be accurately and timely detected, and alarms can be given out according to the faults, automatic detection and early warning are realized; and the jitter fault of the valve of a steam turbine can be detected as early as possible, so that intervention and processing can be performed as soon as possible, and therefore, the electricity quantity of a generator set and the working condition of the steam turbine will not be affected.