The invention discloses a method and apparatus for measuring interhamonics of a power grid. The method comprises the following steps of: tracking the power grid by a PLL (Phase Locked Loop), accurately measuring frequency by a zero crossing detector matched with a DSP (Digital Signal Processor): sampling a power grid voltage (current) waveform at a constant sampling frequency, tracking the power grid frequency by the PLL, wherein the output of the PLL is used as a sampling clock of an ADC (Analog to Digital Converter); and then enveloping an extraction signal by adopting an HHT (Hilbert Huang Transform) method, carrying out FFT (Fast Fourier Transform) analysis on the enveloped signal, and finally figuring out information of harmonics and interhamonics of the power grid. The apparatus comprises three voltage transformers, three current transformers, an approximate synchronous sampling unit and an HHT-FFT computing unit. According to the invention, the HHT method is combined with the FFT method, power frequency carriers and envelopes are separated by using HHT and are respectively subjected to frequency spectrum analysis by using the FFT; and when the method and the apparatus are used in an actual power grid, measured harmonics and interhamonics precision and instantaneity meet the requirements of IEC (International Electrotechnical Commission).