Abstract: Collaborating with the definition of the power factor for the non-sinusoidal waveform, this paper adopted the Fast Fourier Transformation (FFT) to analyze the spectrum of grid voltage and welding current, considered the influence of current harmonics and their phase shifts on the active power, eventually calculated and obtained the accurate power factor of an alternative current spot welding machine. The results showed good coincidence with the theoretical analysis when the power factor changed with the length of the secondary loop. With the same triggering angle of the silicon-controlled rectify-er (SCR), the calculated power factor was smaller than that obtained by the traditional angle θ method, but was identical with the data which was measured by the power system. In conclusion, the proposed method is feasible and effective.