Laser particle size analysis integrated laser, photoelectric and computer technologies has recently become a mainstream method of grain size testing. As particles with different sizes produce scattered light at different angles to the incident laser, the particle size distribution of the sample can be calculated by measuring the intensity of scattered light at different angles. Because of simple operation, rapid test and high precision, it has an important application in sedimentology. A layer of “mottled clay” is widely developed between the Late Pleistocene and Holocene in Quaternary basins in the coastal areas of Guangdong and Fujian provinces. Current research attributes its origin to exposure weathering of Late Pleistocene marine/fluvial deposits during the last glacial maximum. However, our studies find that the mottled clay has no transition in color, structure and composition with its underlying layer and is therefore not formed from weathering. The mottled clay is silty, easily raised by the wind and similar to typical loess. In order to ascertain the character and origin of the mottled clay, three drill cores in the Pearl River delta area were chosen in this study with the method of laser grain size analysis. The results show that grain size composition is characterized by the modal grain size group of coarse silt(10~50 μm), followed by the group of clay grain (<5 μm), both of which are typical particle compositions of an aeolian deposit. All grain size parameters are in accordance with that of an aeolian deposit. Both particle size parameter scatter diagrams and index distribution range of the mottled clay are consistent with typical loess but different from the underlying deposits. The discriminant analysis exhibits an aeolian origin of the mottled clay. The phase analysis also shows that the mottled clay points coincide with the range of typical loess, but has no genetic correlation with its underlying deposits. It is concluded that the mottled clay is not a weathering product of its underlying sediments but an exotic aeolian deposit. This conclusion is of great scientific significance for reconstructing the paleoenvironment of the last glacial period in Fujian and Guangdong coastal areas in the future. The laser particle size method based on optical scattering provides effective scientific evidence for judging the sedimentary environment and origin of the sediments.