Based on new time frequency algorithm, a new interharmonic analysising method is raised in this paper. The number and approximate frequency of interharmonics is firstly gotten using Least-square algorithm. Then new time frequency algorithm is applied to measure the accurate frequency and magnitude of each interharmonic. The simulation indicates that this method mentioned above is able to measure frequency and magnitude of interharmonics accurately and real-timely, with the advantage of high frequency resolution and measuring precision and low calculation amount performance.