Magnetic composite photocatalyst Bi2O3/SrFe12O19 was synthesized by hydrolysis with medium temperature sintering method. X-ray diffraction (XRD) investigation revealed that introduction of SrFe12O19 did not change the favorite growth direction of Bi2O3, [121] orientation. Micromorphology study indicated that SrFe12O19 was distributed on the surface of Bi2O3 to possess the best possibility of forming some heterojunction structures. Vibrating sample magnetometer (VSM) measurements manifested the composite possessed better magnetic properties, which was conducive to its separation, recycling, and reuse. Three main reasons for the increase in photocatalytic activity of Bi2O3/SrFe12O19 were (1) the formation of p–n-type heterojunction between p-type Bi2O3 and n-type SrFe12O19 semiconductors, (2) magnetic field effect stemming from magnetic composite itself generated a shunt effect for photoexcited electrons, and (3) as a sensitizer absorbing visible light, SrFe12O19 could help Bi2O3 absorb more incident photons and then produce more photoexcited electron–hole pairs. In addition, this work was expected to provide a simple preparation method for various functional materials, especially magnetic functional materials.