Jane harmonic detection technology is the important content of power system harmonic governance, this paper proposes an improved ITD method combined with singular value difference spectrum of fast high precision detection. By improving the ITD method can directly according to measured data with feature information contains system parameter identification. First using singular value difference spectrum system order, and then using ITD be improved directly on the noise signal accurately detect the actual frequency of each harmonic, using the Least square method (further Squares, LS) on the amplitude of harmonic component between accurate detection. Improve ITD method with SSI method of covariance matrix (Toeplitz) as the input data, and overcome the data processing method is not accurate, the error of the parameter identification precision is improved. Compared with the method of SSI, improved ITD method accuracy does not decrease at the same time shorten the calculation time. Simulation results show that the method calculating speed, high precision, strong ability to resist noise, verify the feasibility and effectiveness of the proposed method.