Abstract: The diatom mat deposits indicate the blooming and quick deposition of diatoms. They are well developed in the western tropical Pacific during the last glacial stage. The Principal Component Analysis(PCA) of the diatom from the core WPD-03 indicate that the principal component I (PC I) can represent changes in sea surface salinity(SSS), the higher the score, the lower the salinity; whereas the principal component Ⅱ (PC Ⅱ) may indicate sea surface temperature (SST) changes, the higher the score, the higher the temperature. In the late period of MIS3, because of the upwelling of low salinity water, the sea surface salinity was decreased as the nutrients increased. It resulted in the blooming of non mat-forming diatoms. The "low salt cap" will cause the initiation of stratification, and induce giant diatom blooming, which resulted in "Fall dump", thus formed the mat diatom deposits. It may also indicate the input of low salinity seawater into the study area, as one of the reasons for the formation of the diatom mat deposits.