Red pigment of continental red-bed is known originating from the fine-particle hematite in the rocks. Advance of researches on the origin of continental red-bed demonstrates that the red pigment of red-bed originated from its diagenetic but not depositional process. The high diagenetic temperature causes the dehydration of iron hydrate to form hematite, generating the red pigment. For examining the above hypothesis, the authors of this paper designed an experiment to approach the reddening process, i.e. formation of the red pigment of continental red-bed. Black ooze sampled from the Holocene sediments of the Pearl River Delta was heated in different ways. The diffuse reflectance spectroscopy (DRS) of those heated ooze samples were detected with Perkin-Elmer Lamdba 950 ultraviolet/visible/near-infrared spectrophotometer, and moreover, red-values of the samples were calculated for determining their coloring levels. Iron in black ooze sediment is predominantly in the form of goethite. Experimental results verified that initial dehydration-temperature of goethite is about 150 ℃, either enhancing temperature or prolonging heating time is accompanied with decreasing goethite and increasing hematite, and a positive relationship exists between red-value of samples and peak-height of hematite. The experimental results strongly support the idea of thermal origin of continental red-bed.