The global shortage of freshwater is becoming increasingly severe, necessitating the development of green and efficient water treatment technologies to address challenges in high-salinity environments. Inspired by the salt excretion mechanism of marine fish gills, a molecular and interfacial synergistic construction (MISC) strategy was applied to build carboxymethyl chitosan (CMCS) and sodium alginate (SA) all biomass aerogel evaporators with high strength and excellent salt resistance. At the molecular level, the abundant charged and hydrophilic groups endow the aerogel with strong ion-regulation and water-affinity capabilities, giving rise to two interfacial processes: Donnan-based ionic exclusion and a water-film effect that drives salt removal through interfacial flow. With this dual-effect synergy, the evaporator achieved a high evaporation rate of 2.88 kg m−2 h−1 under 1 sun and operated stably for 24 h in 20 wt% saline water without salt deposition. The interpenetrated network of rigid CMCS and flexible SA further provides excellent mechanical properties and structural stability. Importantly, the evaporator is entirely composed of natural biomass materials, allowing for low-cost scalable fabrication with good biosafety and biodegradability. Overall, this work establishes a gill-inspired dual-effect framework for overcoming salt accumulation and offers a sustainable and efficient route for solar-driven desalination under extreme salinity.