In order to improve power quality and accurately measure the frequency values, amplitudes and phases of harmonic and inter-harmonic signal in power grid, a high precision combinational optimization algorithm for measuring electric grid harmonics and inter-harmonics is presented. Firstly, double window all-phase FFT time-shift phase difference spectrum correction method is used to detect the frequency values of harmonic and inter-harmonic signal. Then the objective function of the parameters of amplitude and phase angle is built. The initial value and iterative precision are set. Iterative calculation is performed to target function by quasi-Newton optimization algorithm. Finally the corresponding amplitudes and phase angles are estimated. Using the method to detect the complex inter-harmonic signal, the simulation experimental results show that the algorithm based on the combinational optimization algorithm of double window all-phase FFT time-shift phase difference spectrum correction and quasi-Newton optimization algorithm has realized the higher precision detection of inter-harmonic signal frequency, amplitude and phase three characteristics. And the algorithm has a higher ability to resist noise.