Abstract: The present invention disclosed a method for calculating an oscillation damping ratio of a power grid, and relates to the field of power system operation and control. When a power system oscillates, an oscillation period, two oscillation extreme points, and an oscillation direct current component start to be detected. Then an oscillation decay time constant is calculated according to the two oscillation extreme points and the oscillation direct current component. The oscillation damping ratio is calculated through the oscillation decay time constant and the oscillation period. Compared with a conventional fitting method, the conventional fitting method cannot calculate and analyze the oscillation damping ratio in a cast that a complete oscillation waveform is not obtained. The method proposed by the present inventive patent may complete the calculation of the oscillation damping ratio in a case of obtaining 2 pieces of data, so as to accelerate the speed of calculation.