The invention provides a method for monitoring a low-frequency oscillating source based on PMU data, and relates to safety operation of an electric system. The method comprises: using a phasor measurement unit PMU to obtain information of a bus outlet side of a generator of an electric power system power plant, the information specifically including active output power, reactive output power, voltage phase angles, voltage amplitude values, and power angles of the power plant outlet bus, and according to the obtained data, calculating oscillation energy of the power plant bus, and using a TLS-ESPRIT algorithm to identify the obtained data, extracting a dominant oscillation mode of a power plant generator side, and obtaining corresponding oscillation characteristic parameters. If an oscillation source is in the power plant, oscillation energy flows out of the power plant, an oscillation energy flow is a positive value, and conversely, the oscillation energy flow is a negative value. The method is advantaged in that the PMU can be directly used to obtain input signals and monitor the low-frequency oscillating source of the electric power system in real time, and the method is convenient and fast, and is fast in computation speed, and is good in practicability.