Strontium ferrite has been widely used in magnetic sealing, ferrofluids, and magnetic resonance imaging. To investigate the optical properties of strontium ferrite composite materials in the terahertz (THz) frequency range, we measured the refractive index, extinction coefficient, and other optical constants using a THz time-domain spectroscopy (THz-TDS) system. By analyzing the response signals of THz waves with various polarization states, we compared the changes in polarization resulting from magnetization in the strontium ferrite composites. Additionally, we examined the effect of THz field strength on the optical rotation of the samples. Our findings indicate that both the strontium ferrite composite and its magnetized counterpart exhibit optical rotation under both weak and strong THz field conditions. Notably, the optical rotation effect is more pronounced in the magnetized composite when exposed to a strong THz field. These results provide significant theoretical and experimental insights into the application of strontium ferrite materials in the THz frequency range.