This paper points out that the error of the random independent vector method is due to that the harmonic current at the common point of coupling is approximately equivalent to the customer-side harmonic current. According to the error analysis, the method to assess the ultility harmonic impedence based on the second order blind identification method is proposed. First, the fast-changing parts and slow-changing parts are obtained by filtering from the harmonic voltages and currents measured at the PCC. According to the nature that the fast-changed part of ultilty harmonic current is approximately independent to the fast-changed part of customer-side harmonic current, the four independent components can be calculated by SOBI from the harmonic voltage and current at the PCC. Then the customer harmonic current is picked out using the correlation coefficients, which is because the harmonic current at the PCC is approximately equal to the harmonic current of customer, i.e., the correlation coefficient between the harmonic current at the PCC and the harmonic current of customer is much larger than the correlation coefficient between the harmonic current at the PCC and the utility harmonic current. Finally, based on the nature that the random vector covariance is zero, the utility haromnic impedence is attained. Through the simulation calculation, it is easily seen that the method effectively improves the calculation precision of the random independent vector method, which has high accuracy.