Ionic thermoelectric material is gaining increasing attention, and many attempts have been devoted to obtain huge ionic thermopower, which remains a big challenge. Herein, the idea of “ion thermal migration channel” was propose, and a high ionic thermopower was achieved by regulating the thermal migration channel in cellulose hydrogel system. It is found that the channels size as well as the channel-ion interaction played dominant roles. Initially, the channel size in cellulose/KCl hydrogel were optimized to enlarge the channel-Cl− interactions via counter-ion condensation effect, which improved the thermopower from 2.45 mV·K−1 to 8.13 mV·K−1. Then, sodium alginate as polyanion was doped and multi-valance ion cross-linking was carried out, to further enhance the channel-Cl− interaction, and a high thermopower of 22.09 mV·K−1 was realized. In addition, the cellulose based ionic thermoelectric hydrogel displayed viable basis for energy harvesting and temperature sensing as wearable flexible devices. The present work not only provides a systematic strategy for the thermopower regulation, but also supplies an applicable approach for the preparation of sustainable thermoelectric materials with biomass resources, demonstrating great potential in sustainability and green energy.