To alleviate the global freshwater crisis, a self-floating magnetic Janus biomass composite hydrogel has been fabricated through a rational asymmetric bilayer structure design, which exhibits many merits such as remarkable light-absorption, photothermal conversion, water-evaporation, salt rejection, wind-resistant, etc. Specifically, sodium alginate, cellulose nanofibrils (from corn straw), and acrylic acid were crosslinked by triple interpenetrating networks (chemical cross-linking, ionic cross-linking and hydrogen bonding). Janus hydrogel evaporator was constructed by incorporating the top layer containing light-absorbing core–shell magnetic nanoparticles (Fe3O4@PDA NPs), and the bottom layer composed of self-floating foam gel due to the addition of foaming agent. Under one solar irradiation, the average evaporation rate and photothermal conversion efficiency of Janus biomass composite hydrogel reached 2.89 ± 0.09 kg·m−2·h−1 and 95.66 ± 2.68 %, respectively. Its evaporation stability and salt tolerance capability have been demonstrated by extended period of simulated desalination. Furthermore, Janus biomass composite hydrogel also had excellent mechanical property (ultimate stress of 1.6 MPa) and magnetic performance, which can not only effectively resist the stress induced by wind and waves, but also enable the magnetic maneuvering and wind-resistance floating on the sea. When the input voltage of electromagnet was 24 V, the furtherest response displacement and maximum resisted wind speed were 9 cm and 7.2 m/s, respectively. Therefore, such magnetic self-floating Janus biomass composite hydrogel serving as interfacial solar-driven evaporator system offers a green, efficient and sustainable solution for clean freshwater production.