This paper chose the lacustrine sediment as study object which was near the subsidence center of Subei Plain. Based on X-ray diffraction (XRD) analysis, the clay mineral components and their assemblage feature of one core sediments (YZ1) in Subei Plain were measured. The characteristics and environmentical significance of clay minerals were analyzed. The results show that the main clay minerals in YZ1 core sediments were illite, kaolinite, chlorite and smectite, and feldspar and quartz piece were minor. The palaeoclimate of the study area has experienced three stages: 13 770~11 600 cal a B.P., 11 600~9 610 cal a B.P., and 9 610~1 520 cal a B.P. Crystallinity also showed relatively low, suggesting a relatively warm and humid palaeoclimate.