Sedimentary grain size is widely used as an indicator of lake water depth,surface runoff intensity,and climatic changes, etc. However, as different lakes have variable topography and hydrological processes, the climatic/environmental significance of sedimentary grain size is variable. In this paper, we studied the relationship between sedimentary grain size and water depth at Lake Chenghai ( 26°27'~ 26°28'N,100°38' ~ 100°41'E), northwest Yunnan Province, Southwestern China. We determined the sedimentary grain size (1 cm, 5 cm and ~(137)Cs-peak layer) and the carbonate content (2 cm, 5 cm and ~(137)Cs-peak layer) of surface sediments in the cores collected at different water depth in Lake Chenghai, and studied the spatial correlation between grain size and water depth. We also examined the relationship between grain size of a vertical-down core and the observed lake level variations during the past 50 years. The results show that there is a significant inverse spatial correlation between the sedimentary grain size and water depth, i.e.,coarser grain size correlates to shallower water depth, and vice versa. This spatial grain size-water depth relation is verified by the spatial relationship between water depth and sedimentary carbonate contents. The temporal relationship between grain size and water depth is similar to that on spatial scale. We propose that the spatiotemporal relationship between grain size and water depth at Lake Chenghai is related to the variations in distance from the sampling sites to the lake shoreline/riverine estuary caused by changes in lake areas (or lake levels). We further verified the relationship between sedimentary grain size and historical lake levels during the past millennium. The sedimentary grain size was coarse during the warm periods, like the Medieval Warm period (MWP) and the modern warm period,while it was fine during the Little Ice Age (LIA),indicating lower lake levels during MWP and modern warm epoch but higher lake levels during the LIA, which is consistent with the lake levels revealed from beach evidence and those inferred from historical literatures.