Abstract: Climatic and environmental changes have rendered great impacts on the migration and evolution of hominid in Southeast Africa, since the Marine Isotope Stage 12(MIS12). Clay mineral assemblages, contents and mineralogy of 149 sediment samples collected from the Hole U1474 by the Expedition 361 of the International Ocean Discovery Program(IODP), have been analyzed and measured with X-ray diffraction(XRD). The hole is located in the Natal Valley of Southeast Africa, The results show that the clay minerals are mainly composed of smectite(39.23% on average), illite (26.11% on average), kaolinite(17.79% on average)and minor chlorite(17.19% on average). The crystallinity of illite in all samples are high and on an average of 0.35°Δ2θ(＜0.4°N 2θ), and the illite chemical indices are as low of 0.30(＜0.43 on average. The clay mineral assemblages of the Hole U1474 suggest a riverine source mainly derived from the three major rivers (the Tugela River, the Limpopo River and the Zambezi River)in Southeast Africa. The variation of clay mineral composition and related parameters of the Hole U1474 indicates that the climate changes in the Southeast Africa since MIS12 is obviously characterized by glacial-interglacial cycles and can be divided into five stages. Each stage is cold and dry during the glacial period, and relatively warm and humid during the interglacial period. In each period, there are some abnormal suborbital climate fluctuations, such as cold and warm, dry and wet fluctuations, affected by regional atmospheric circulation and adjacent ocean currents, such as the Agulhas Current.