A method and system for quickly troubleshoot power oscillation cause by abnormal excitation system is disclosed. The implementation steps of the method include collecting data parameter during power oscillation of a generator excitation system, performing multi-round diagnosis analysis according to the data parameters of the power oscillation period, so as to judge whether the power oscillation iscaused by the generator excitation system abnormality, and whether is caused by the power system stabilizer PSS output abnormality, the AVC command value abnormality, the AVR set value abnormality, and the sampling channel abnormality; The system includes a computer system programmed to perform the quick troubleshooting method described above. As that invention does not need to carry out laboratory simulation and field test, the invention can quickly check whet the power oscillation is caused by the abnormality of the excitation system and each component thereof, and has the advantages of simplifying the analysis of the power oscillation event, shortening the outage time of the generator set, improving the efficiency, reducing the cost and being wide in adaptability.