With wide application of nonlinear loads, the problem of harmonics in the grid is increasingly serious. It has great significance in measuring and analysing harmonics accurately. To reduce complexity of harmonic analysis in symmetrical three phase circuit, method of harmonic components analysing based on synthetic curve periodicity characteristics of harmonic signal on alpha-beta plane is proposed. Firstly, alpha-beta transform to single phase electrical parameters is done, at the same time amplitude and periodicity of synthetic curve are calculated. Then, according to periodicity of synthetic curve, harmonic components existing in circuit possibly are analyzed. Ultimately, combining with information such as total harmonic distortion of harmonic signal, harmonic ratio of existing harmonics is calculated. Analysis shows that when periodicity of synthetic curve amplitude is less than a quarter of power frequency periodicity, calculation amount of the method is only about two-thirds of fourier analysing method. Calculation of cases testifies the availability of proposed method.