The harmonic detection technology of active filter needs to separate the fundamental component of three-phase current. Under the Non-ideal Conditions of Power Grid, due to the influence of negative sequence fundamental component of current, the performance of traditional harmonic detection technology is affected to some extent. In this paper, the frequency domain analysis of sinusoidal integrator under asymmetrical condition of power network is presented, A harmonic detection technology under non-ideal condition of power grid is proposed. The method used in the αβ coordinate sinusoidal amplitude integrators (SAI) selection frequency, three-phase current fundamental component extraction. In this paper, the working principle, mathematical model and frequency domain analysis of harmonic detection technology based on sinusoidal integrator are introduced in detail, and the feasibility of the proposed method is analyzed theoretically. Building MATLAB/simulink models, The simulation results show that the proposed method can effectively eliminate the influence of non-ideal working conditions on harmonic detection. The fundamental component is detected accurately and quickly and the compensating current instruction is given.