Abstract: The high proportion of renewable energy and power electronic equipment has generated a large number of sub-/supersynchronous interharmonics in the power grid, resulting in significant deviations in the fundamental frequency measurement of synchrophasor measurement unit（PMU）, which affects the effectiveness of various frequency applications. Therefore, the paper proposes a mechanism analysis and suppression method for fundamental frequency measurement error under the influence of sub-/super-synchronous interharmonics. Firstly, a frequency domain characteristic model of the frequency measurement algorithm is established, and based on this, a mathematical model of fundamental frequency oscillation under the influence of a single interharmonic and symmetrical sub-/super-synchronous interharmonics is derived, revealing the mechanism of fundamental frequency measurement error under interharmonic interference. Secondly, based on the oscillation behavior of frequency, a fitting model of multi-component superposition is established, and a short-time window frequency oscillation component suppression method based on Fourier transform and least squares method is proposed, which significantly reduces the fundamental frequency deviation. Finally, simulation and on-site recorded data test verify that the proposed method reduces frequency measurement errors greatly.