Abstract: The wind farm side harmonic impedance could not be neglected when assessing the wind farms harmonic emission level. Because of the low impedance characteristic of the wind turbine filter, the wind farm side harmonic impedance is small. This paper presents a new method to estimate the wind farm harmonic emission level at the point of common coupling（PCC）based on improved covariance characteristic of random vectors. According to the topological structure of the wind farm, the equivalent harmonic impedance of the wind farm is calculated, with the measured data at PCC the wind farm harmonic source is obtained. Since the system side harmonic source and wind farm side harmonic source are independent of each other, the background harmonic related term can be eliminated by using the covariance characteristic, and the system side harmonic impedance and then the wind farm harmonic emission level can be calculated. Considering that the topology of the wind farm is complex, the impedance approximation of the power collecting line network is proposed to simplify the calculation of the wind farm side harmonic impedance. Theoretical analysis shows that the traditional approach which is based on the approximate independence of the harmonic current at PCC and the background harmonic ("approximate independent method") is in error，because of the weak connection between the independent vectors. Based on the result of simulation and measured data, it verifies that the proposed method is more effective to eliminate the background interference than "approximate independent method" , and the proposed method is also effective when the background harmonic ratio is high.