Solar-driven interfacial evaporation is a sustainable and economical technology for fresh water generation. Structural design of photothermal material is an effective strategy to improve the evaporation performance but usually bothered by complicated processes and non-adjustability. Herein, magnetic nanoparticles assembled photothermal evaporator was developed, which showed an adjustable spinal array surface under uniform magnetic field induction. By regulating position in the magnetic field, the desirable surface structures could be uniform at relatively low load density of magnetic nanoparticles to improve light absorption via multiple reflection. Magnetic field induced evaporator could accelerate evaporation to over 1.39 kg m^-3 h^-1 under 1-sun illumination, which was 2.8 times that of natural evaporation. After coated by carbon layer, magnetic nanoparticles could overcome the oxidation to realize stable evaporation in long-term desalination. The facile strategy to optimize the surface structure via magnetic field is appropriate for various fields with special requirements on surface structure.