We propose that half-quantized helical hinge currents manifest as the fingerprint of the axion insulator. These helical hinge currents microscopically originate from the lateral Goos-Hänchen (GH) shift of massless side-surface Dirac electrons that are totally reflected from the hinges. The helical current induced by the GH shift is half-quantized. Semiclassical wave packet analysis uncovers that the hinge current has a topological origin and its half-quantization is robust. Lastly, we propose an experimentally feasible six-terminal device to identify the half-quantized hinge channels by measuring the nonreciprocal conductance.