To understand the mechanism of magnetic variation of red earth in southern China, the authors investigated magnetic characteristics, characteristics of magnetic minerals and degree of weathering of Jiujiang red earth by using magnetic susceptibility instrument, scanning electron microscope and X-ray fluorescence spectrograph. The results indicate that the upper sandy clay has experienced weaker weathering than the lower net-like red earth, but it has a higher magnetic field strength than the lower net-like red earth. The increase in magnetic strength is closely related to the content of superparamagnetic particles and multidomain magnetite. Increasing weathering results in the destruction of the magnetite crystals and the transformation of ferromagnetic minerals to weakly magnetic minerals, which is the main reason causing a lower magnetic strength of net-like red earth than magnetic strength of the sandy clay. The results also indicate that multidomain magnetite in red earth is not formed in weathering and pedogenesis but in the weathering residue from the parent material of the red earth. From middle Pleistocene to late Pleistocene, the climate changed from hot and humid to cold and dry and weathering intensity gradually decreased, resulting in more ferromagnetic minerals in the upper sandy clay than in the lower net-like red earth. Therefore, the main magnetic-bearing minerals in red earth are not the products formed in weathering and pedogenesis, and magnetic characteristics of red earth can't reveal the climate change.