The new harmonic and interharmonic measurement method with good measuring accuracy for non-stationary signals under synchronous sampling is recommended by IEC 61000-4-7:2002 standard, but it shows poor measuring accuracy under asynchronous sampling, and the results are variable when relative phase of harmonic and interharmonic changes. To make up the deficiency above, this paper presents an advanced method which combines IEC standard framework with all-phase FFT. The method has the excellent property of inhibiting spectral leakage and the feature of phase invariance of all-phase FFT for detecting various typical non-stationary signals, and then is compared with the IEC method and IEC order cosine window method. Simulation results demonstrate that the advanced method is suitable for harmonic and interharmonic measurement for asynchronous sampling, even though the relative phase of harmonic and interharmonic changes, the measurement results are still with high accuracy and keep constant.