We have performed experimental and theoretical studies on the influence of ac magnetic field amplitude on the magnetoimpedance tensor in an amorphous wire with helical magnetic anisotropy. For the experimental measurements, we used an amorphous wire of composition (Co0.94Fe0.06)72.5Si12.5B15 with negative, nearly zero magnetostriction constant, excited either by an ac circular hφ or by an axial hz magnetic field created by an ac electric current. We changed the ac current amplitude from 7.5 to 40 mA and the current frequency f from 1.5 to 20 MHz. The values of the asymmetric giant magnetoimpedance ratio associated with the sweeping direction of the dc field Hex and the corresponding sensitivity were 211% and 0.64 V/Oe, respectively, for an ac current of 37.5 mA at 3 MHz. For the theoretical study based on the magnetization rotation, we obtained the second-order harmonic of the ac magnetization m&oarr;(2) induced by the relatively high ac magnetic field by solving the Landau-Lifshitz-Gilbert (LLG) equation. We also considered a second-order surface impedance tensor ςˆ(2), which allowed us to analyze quantitatively the influence of the ac magnetic field amplitude on the impedance tensor of the wire. We obtained the domain model of the wire with helical magnetic anisotropy having multidomains and the magnetization vector ±M0 directed in the easy direction, and the corresponding static magnetic configurations, by solving the static LLG equation. For the given magnetic configurations, we calculated the second-order impedance tensor ςˆ(2). The results can well explain the irregular field characteristics of the voltage responses at low dc field value, when the wire was excited at high frequency and at large ac magnetic field.