In this study, we propose and demonstrate an all-fiber orbital-angular-momentum (OAM) mode encoding system, where through helical fiber gratings (HFGs), binary symbols are encoded to or decoded from two OAM modes with topological charges (TCs) of $-1$ and $+1$, respectively. We experimentally validate that the OAM mode generated by a clockwise-helix HFG (cHFG) can be converted back into fundamental mode by using an HFG with a helix orientation opposite to that of the cHFG, i.e., ccHFG. Benefited from utilization of the HFGs, the proposed OAM mode encoding system has a low cost, low insertion loss, high mode conversion efficiency, and polarization independence. To the best of our knowledge, this is the first demonstration of the HFGs-based all-fiber OAM mode encoding/decoding scheme, which may find potential applications in optical communication and quantum communication as well.