Flexible hydrogel thermocells (TECs) harvesting low-grade thermal energy offer innovative solutions for powering wearable devices. However, achieving high toughness and significant thermopower in flexible hydrogel TECs remains challenging. This study presents an efficient heat harvesting method utilizing temperature-induced phase transition in the cross-linked poly(N-isopropylacrylamide-co-acrylamide) (p(NIPAAm-co-Am)) hydrogel to regulate ion diffusion and thermoelectric performance. By adjusting the proportion of copolymer monomers, the hydrogel exhibits tunable lower critical solution temperature of 31.6–40.5 °C. The p(NIPAAm-co-Am) TEC with [Fe(CN)6]3-/4- achieves a Seebeck coefficient of 2.6 mV K-1 and a tensile strength of 580% at the molar ratio of NIPAAm/AAm being 9:1 and the concentration of [Fe(CN)6]3-/4- being 0.30 M. In addition, four p-type hydrogel legs are electrically connected with copper tape in series, and the fabricated TEC exhibits an optimum output voltage of 58 mV and an output power density of 39.3 μW m-2 at ΔT = 10 K. As an application demonstration, the TEC not only illuminates the LED through an amplifier under the temperature difference between the human body and the environment, but also effectively monitors body temperature. Therefore, this work provides a novel idea for constructing thermosensitive TECs with enhanced thermoelectric performance, which has potential application in powering wearable electronics and monitoring body temperature.