The Helmholtz coil constant (k _h) is a crucial standard in magnetic moment measurement devices for permanent magnet materials. To overcome the problem of lowaccuracy of the direct-current (DC) calibration method, this study used a constant sinusoidal current in the Helmholtz coil and measured the induced voltage of the detection coil with known coil turns and coil area. Subsequently, the k _h was calculated. The noise signal deduction rate in the induction voltage of the detection coil was greater than 99%, its influence on the induction voltage is less than 0.005%, and the repeatability of the calibration results is 0.003% (1δ). The results reveal that the alternating current (AC) method and orthogonal calculation (OC) can accurately measure the valid values of the voltage signal under the influence of the spatial stray field during the calibration of k _h.