In traditional solar desalination systems, the solar collector and demineralizer are separate components with a large thermal resistance and pipeline heat loss between them. This paper introduces the concept of a floating solar desalination film, which uses concentrated light to directly evaporate seawater. The greatest advantage of this design is that it combines solar energy collection and seawater demineralisation as an integrated system, eliminating pipeline heat losses and realising large-scale desalination on the surface of the sea, saving space on the land. This paper describes the operating principle of the floating solar desalination film, carries out an optical simulation for the light concentration process, analyses heat and mass transfer processes for theory calculation in the film's microstructure, and investigates optimisation of the device structure. A performance test was conducted on the device, showing water production and temperature measurements at different weather conditions. In addition, the average daily efficiency of the system was calculated. According to the test results, with comprehensive tracking of the sun, the water production per unit area of this test device reached 1.38 kg/m(2).day. With azimuth-only tracking of the sun, the water production was 1.19 kg/m(2).day, and the daily solar-thermal conversion rate reached 22.7%.