Solar powered clean water production has been considered a favorable way to address the problem of the global water shortage. Recently, the interfacial solar-steam generation system has greatly improved water evaporation by localizing the solar energy at an advanced solar-thermal conversion material interface. However, the specific water productivity (SWP) is still far away from a satisfactory level due to the strong mutual interference between the incident sunlight and the generated water vapor, which causes a huge loss in energy and in turn restrains the final efficiency in water evaporation and collection. SWP is the water collection per solar radiation area per hour, which reflects the actual solar efficiency for water production and a key concern in desalination. Herein, we report a rational Janus-interface solar-steam generator (J-SSG), which separates the water evaporation and the solar-thermal conversion on the two sides of the film generator. This J-SSG demonstrates a water evaporation rate of up to 2.21 kg m(-2) h(-1) under 1 sun in a large area of 100 cm(2). More importantly, a record high SWP of 1.95 kg m(-2) h(-1) is realized in a simple system, and the SWP efficiency corresponding to the ratio of SWP to evaporation rate is as high as 88%. In an outdoor test (Beijing, solar energy similar to 15 MJ m(-2) day(-1)), 10 L purified water per square meter have been easily achieved. This Janus-interface engineering of the solar-steam generator provides a novel strategy and solution for solar powered water production of practical significance.