Magnetic-particle photocatalyst with a core-shell structure takes a ferromagnetism particle, a paramagnetism particle or a diamagnetism particle as a core, and photocatalyst as a shell. The ferromagnetism, paramagnetism or diamagnetism particles are gamma-Fe2O3, SiO2 or MnO particle respectively, and the photocatalyst is Gd3-xBixSbO7, Gd3-xYxSbO7 and In3-xBixTaO7 respectively, wherein the grain diameter of the gamma-Fe2O3, SiO2 and MnO is 80-2,000nm. Water-treatment is carried out on the photocatalyst in a magnetic field-photocatalytic system, and the periphery of a reaction system for water-treatment is provided with an alternating magnetic field generator with adjustable magnetic field intensity, wherein the range of the magnetic field intensity is 0-15T, and radiation light sources of the reaction system adopt a 300W xenon lamp and a 400W high-voltage mercury lamp; the ferromagnetism particle, paramagnetism particle and diamagnetism particle respectively account for 33+/-10% in themagnetic-particle core for coating the photocatalyst, and are completely and uniformly dispersed in aqueous solution, thus the composite photocatalyst is uniformly distributed in the upper, middle and lower layers of the aqueous solution.