A method based on fast approximate joint diagonalization(FAJD)is proposed to estimate the system-side harmonic impedance and calculate the harmonic emission level. Under Cartesian coordinates

according to the basic principle of harmonic Norton equivalent circuit

the relationships between the harmonic voltage and current at the point of common coupling(PCC)and the system-side and user-side current of the harmonic source are deduced. Considering basic conditions that both sides of harmonic current source are independent

the FAJD method is used to obtain the mixed matrix and the both sides' components of harmonic current source. Then the real and imaginary parts of the system-side harmonic impedance are obtained by comparing the basic circuit equation after getting mixed coefficient matrix by the least squares method. Hence the harmonic emission level is assessed. The method is based on the measured harmonic voltage and current at point of common coupling(PCC)

without matching priori data or requiring observation target matrix to be an orthogonal matrix. So it has higher separation accuracy and better performance of anti-interference of background harmonic without pre-whitening while avoiding introducing unnecessary errors. Finally

based on the result of simulation and measured data

the feasibility and effectiveness of the proposed method is verified.