In order to improve the success rate of adaptive reclosure and minimize the incomplete phase operative time, a method based on flatness characteristics of allphase FFT (apFFT) spectrum is presented to detect the arc extinction time for transient singlephase earth faults in transmission lines with shunt reactors. The apFFT spectrum of the terminal voltage of faulty phase is analyzed. It is found that, before the secondary arc extinguishes, the apFFT phase spectrum is not smooth, because of outstanding variation of highfrequency components caused by the highly nonlinear transition resistances. After the secondary arc extinguishes, the apFFT phase spectrum is flat, because the transition resistance disappears and highfrequency signals change smoothly. Utilizing the difference of the flatness characteristic in highorder harmonic apFFT phase spectrum before and after the secondary arc extinguishes, a reliable method is proposed to identify the arc extinction time accurately and effectively. Simulations using ATPEMTP verify the correctness and effectiveness of the proposed method. The proposed method is almost insusceptible to fault location, transition resistance and power angle. The validity of the method is verified by the analysis of the actual wave data.