The weathering universally takes place in red-bed mudstone slope，which is not completely solved till now. Under the influence of geotechnical weight force, vibration, and other factors, unsteady red-bed mudstone slope caused due to weathering often lead to harmful deformation and failure, resulting in collapse of geological disasters such as landslides, falling rocks and mudslides. Weathering depth is the vertical distance after the rock burst produced inside the fracture surface micro cracks or micro-cracks in the weathering zone extends to the interior of the rock. Weathering depth can provide an important basis for red-bed mudstone slope treatment. The rock core was obtained by using the core drilling machine in the typical red-bed mudstone slope. Terahertz time-domain spectroscopy is a new spectroscopy approach to characterize material based on terahertz pulse. Terahertz time-domain spectroscopy is used to measure the terahertz transmission spectroscopy of sample obtained. The THz transmission spectra of samples are being analyzed. Only minor differences are found among the transmission spectra of different samples, and there are not characteristic absorption peaks been observed. To obtain weathering depth of sampling site, an intelligent and efficient SVM model of the transmission spectra of samples is employed to distinguish and predict the depth. Compared with the actual depth, the relative error is less than 7.09%. The results showe that THz spectroscopy based on support vector machine (SVM) model can effectively distinguish samples and the predict depths, which can provide an important reference for the red-bed mudstone slope treatment.