Coupling low-grade heat (LGH) with salinity gradient is an effective approach to increase the efficiency of the nanofluidic-membrane-based power generator. However, it is a challenge to fabricate membranes with high charge density that ensures ion permselectivity, while maintaining chemical and mechanical stability in this composite environment. Here, we develop a bis[2-(methacryloyloxy)ethyl] phosphate (BMAP) hydrogel membrane with good thermal stability and anti-swelling property through self-crosslinking of the selected monomer. By taking advantage of negative space charge and three-dimensional (3D) interconnected nanochannels, salinity gradient energy conversion efficiency is substantially enhanced by temperature difference. Theoretical and experimental results verify that LGH can largely weaken the concentration polarization, promoting transmembrane ion transport. As a result, such a hydrogel membrane delivers high-performance energy conversion with a power density of 11.53 W·m⁻² under a negative temperature difference (NTD), showing a 193% increase compared with that without NTD.