The invention discloses a method for distinguishing free oscillation and forced oscillation of an electric power system, and belongs to the technical field of electric power system analysis. The method includes the steps: monitoring active power amplitudes of a generator, a transformer and a circuit and analyzing active power data with the maximum amplitude after the system starts oscillation; solving the maximum value and the minimum value of the active power in each oscillation cycle, subtracting the minimum value from the maximum value to obtain an oscillation peak-to-peak value and analyzing a few or a dozen of oscillation cycles with peak-to-peak values increased with time from oscillation starting time; subtracting a previous peak-to-peak value from each peak-to-peak value to obtain the increment of each peak-to-peak value and solving natural logarithms for the increment; and linearly fitting the natural logarithms for the increment of the peak-to-peak values, calculating a slope index and judging that oscillation is free oscillation when the slope index is larger than zero and the oscillation is forced oscillation when the slope index is smaller than zero. By the method, oscillation types are distinguished according to waveform features at an oscillation starting stage, oscillation type distinguishing speed and accuracy are improved, a power grid operator can be helped to timely and accurately distinguish the oscillation types, and targeted measures are taken to calm down oscillation.