  One-dimensional (1D) gapless hinge states are predicated in the
three-dimensional (3D) higher-order topological insulators and topological
semimetals, because of the higher-order bulk-boundary correspondence.
Nevertheless, the topologically protected property of the hinge states is still
not demonstrated so far, because it is not accessible by conventional methods,
such as spectroscopy experiments and quantum oscillations. Here, we reveal the
topological nature of hinge states in the higher-order topological semimetal
Cd3As2 nanoplate through spin potentiometric measurements. The results of
current induced spin polarization indicate that the spin-momentum locking of
the higher-order hinge state is similar to that of the quantum spin Hall state,
showing the helical characteristics. The spin-polarized hinge states are robust
up to room temperature and can nonlocally diffuse a long distance larger than 5
{\mu}m, further indicating their immunity protected by topology. Our work
deepens the understanding of transport properties of the higher-order
topological materials and should be valuable for future electronic and
spintronic applications.
