Recently, ionic thermoelectric hydrogels have attracted much attention, and it is desirable to use ionic thermoelectric hydrogels to couple thermoelectric properties and strain sensing performance, enabling potential applications in the field of wearable electronics. Nevertheless, simultaneously improving the Seebeck coefficient and ionic conductivity of ionic thermoelectric hydrogels remains a challenge. Here, a dual-cation doping strategy is used to regulate ion diffusion rate to improve the thermoelectric properties of ionic hydrogels, and a series of poly-(vinyl alcohol) (PVA)-based hydrogels doped by dual cations (i.e., hydrogen ions and alkali metal cations, such as Li+, Na+, and K+) are prepared by a facile cyclic freeze–thaw method. With dual-cation doping, H+ and alkali metal cations interact with the hydroxyl on PVA chains, resulting in partial destruction of hydrogen bonding, which is beneficial for improving ion diffusion rate. The results show that PVA/HCl/NaCl hydrogels demonstrate a high Seebeck coefficient of 7.43 mV K–1 and a good ionic conductivity of 33 mS cm–1 at ambient temperature, which are much higher than those of the PVA/NaCl hydrogel. Furthermore, the PVA/HCl/NaCl ionic hydrogels exhibit good tensile strength (0.65 MPa) and sensitivity (GF = 1.25), making them suitable as flexible strain sensors to monitor body movement, with potential application in the field of wearable electronics.