The invention relates to a conjugated polymer nanoparticle photocatalyst as well as a preparation method and application thereof, belonging to the technical field of polymer photocatalysts. The preparation method comprises the steps of rapidly injecting 1-20mL of a tetrahydrofuran solution of a conjugated polymer and a functionalized polymer into 5-100mL of water under the action of ultrasonic waves, and continuously carrying out ultrasonic treatment for 1-5min; under the protection of inert gas, heating the solution up to 80-100 DEG C for removing tetrahydrofuran, then continuously heating and concentrating the solution to 2-20mL, and filtering the concentrated solution with a 200-220nm of filtering head for removing large particles so as to obtain the conjugated polymer nanoparticle photocatalyst, wherein the size range of conjugated polymer nanoparticles obtained by adjusting the concentration and injection volume of an initial solution is 5-10nm. The preparation method has the advantages of being simple and convenient to operate and fewer in steps; the photocatalytic material is low in preparation cost, environmentally-friendly in preparation process and metal-free; the conjugated polymer nanoparticle photocatalyst has good optical adsorption properties in an ultraviolet visible region and can be used for purifying the environment by using sunlight, thus having wide application prospect.