Surface quality is an important indicator for evaluating the quality of hot-rolled coil steel, which directly affects the service life and performance stability of hot-rolled strip steel products. Dense mill scale defects form on the surface of the steel substrate during the production process of hot-rolled strip steel, and their composition is mainly composed of iron oxides. To ensure the quality of subsequent coating and plating processes, understanding the thickness information of mill scale on the surface of hot-rolled strip steel can effectively reduce the risk of insufficient or excessive pickling. Because terahertz waves are located between infrared and microwaves in the electromagnetic spectrum, and reflect with minimal attenuation on the surface of polar materials such as metals, terahertz time-of-flight tomography (TOFT) meets the technical requirements of non-destructive characterization of micron-level covering layers of steel-based materials. Owing to the difficulty of non-destructive mechanical stripping, the optical properties of the mill scale on the surface of hot-rolled steel strip in the terahertz frequency band are preliminarily estimated based on existing values in the published literature, rather than values obtained through experiments, as a result of which, unexpected errors between the scale thickness calculated based on terahertz results and the nominal value. To accurately determine the thickness distribution of iron scale, hematite (Fe₂O₃) and magnetite (Fe₃O₄) powders were thoroughly mixed with polyethylene (PE) and pressed into pellets. Transmission experiments were conducted on the Fe₂O₃/PE and Fe₃O₄/PE pellets using terahertz time-domain spectroscopy (THz-TDS), and the optical parameters of Fe₂O₃ and Fe₃O₄ inclusions at 10% mass fraction in the frequency range ［350 GHz~2.5 THz］were then accurately calculated using the Maxwell-Garnett effective medium theory and Vegard's law. The refractive indexn, absorption coefficientα, and conductivity σ of Fe₂O₃ and Fe₃O₄ at 1 THz are 3.96 and 5.18, 10.13 and 25.58 cm^-1, and 1.2 and 4.97 S·m^-1, respectively. The method outlined here will also be of interest to a range of powder materials that may need to evaluate their optical properties in the THz band. The research results have laid a theoretical foundation for the future implementation of terahertz-based technology in online monitoring of the quality of hot-rolled steel products within the complex steel production environment, providing a non-contact and non-destructive method. This has important engineering practical significance for promoting the widespread application of terahertz technology.