Abstract: This paper analyses and proposes the definition of power factor of making-breaking capacity of low-voltage electrical apparatus. The power factor(cosφ) is one of the main characteristic parameters of the AC making-breaking capability test circuit, Even if the test current period component is the same, The peak value of test current, the recovery voltage and arc energy at the moment of quenching vary greatly at different cosφ. So far, the IEC and the National standard have specified the value of cos φ, the allowable error and the recommended measurement method, but none of the methods can accurately measure cosφ in the full range of the test circuit. In this paper, various measurement methods of test circuit cosφ are analyzed and discussed, and the phase Angle difference method and DC component method suitable for power factor measurement in laboratory are emphatically analyzed and studied according to the measurement data in laboratory.