The invention discloses a method for synthesizing a novel visible light active Ca-Bi-O light catalyst by adopting the soft chemistry-hydrothermal technology, which has the characteristics that the preparation temperature is low, the application process is simple, and the obtained light catalyst has big specific surface area and high visible light activity and contains Ca-Bi-O compound taking the monoclinic CaBi2O4 as the main body. Transparent solution with a certain concentration is prepared respectively by Ca(NO3)2 and Bi(NO3)3, the Ca(NO3)2 solution and the Bi(NO3)3 solution are mixed uniformly, and dilute ammonia water and the mixed solution are simultaneously dipped into de-ionized water for mixing and keeping a certain pH value, so as to obtain white suspension. The generated suspension is centrifugally separated and diluted into while precipitate suspension, and the while precipitate suspension is placed into a closed kettle made of stainless steel material and lined with white polytetrafluoroethylene for heat treatment, wherein the temperature is 160-240 DEG C, the processing time is 2-10h, and then, after the processing, the Ca-Bi-O light catalyst can be obtained by cooling and drying.