The invention discloses a composition, a film, a preparation method and a detection method for detecting the intensity of an ultraviolet light, and relates to the technical field of optical detection, aiming at detecting the intensity of large area of ultraviolet light as well as lowering complexity in the detection. The composition includes a nematic phase mixed crystal, a chiral additive, a cholesteric liquid crystal, an azobenzenes monomer, a light polymerable monomer and a photoinitiator. The film includes the composition provided by the technical solution. Upon preparation, each component of the composition is evenly mixed and spread to form a mixed system pre-fabricated film layer which is radiated by ultraviolet light, thus preparing the film. According to the invention, the composition is intended for detecting the intensity of the ultraviolet light.