Objectives: Operated by multiple sedimentary processes in some sedimentary environments and dynamics, the occurrence frequencies of different diameter particles, called grain- size distribution (GSD) in sedimentology and geology, record original sedimentary information. Superposed by multi- subpopulations, the corresponding frequency curve of GSD could be bimodal or multimodal. Traditional methods for sedimentological analysis do not research the subpopulations in GSD deeply. After unmixing subpopulations from GSD, usual probability distribution models are unable to calculate statistical parameters of subpopulations thoroughly. Methods: Taking 214 GSDs from modern sediments in the Poyang Lake for example, skew normal probability distribution is used to decompose total 977 subpopulations in this paper. Statistical parameters of subpopulations, such as mean, sorting, skewness, kurtosis, percentage in GSD and maximum frequency, are all calculated. Similarities and differences of these parameters from various sedimentary environments are compared. Results: The statistical parameters of subpopulations are in obvious rules. Means of subpopulations focus in six intervals: 0~1, 1~2, 2~3, 4~5, 6~7 and 7~8. Subpopulations with means in 0~2, 2~3 and 4~5 are excellent, excellent—well and excellent— moderate sorted respectively; sorting of subpopulations with means in 6~8 are negative linear correlation with means. Subpopulations with means in 4~5 and 6~8 are mainly very coarse skewed. Kurtosis of all subpopulations are less than 0.8. Percentages in GSDs of subpopulations with means in 4~5 and 6~8 are generally less than 30%. Maximum frequencies of all subpopulations are less than 12%, and that of subpopulations with means in 4~5 and 6~8 are less than 3% and 2% respectively. Percentages in GSDs of subpopulations with different means are positive linear correlation with corresponding maximum frequencies. Conclusions: From the channel to terminal of river, and downstream or left and right away from channels in the river terminal, means of main subpopulations decrease gradually. Percentage of each subpopulations in GSDs of sediments from interchannel hollows and near lake regions are approximated, which means that there is no obvious main subpopulation. Reworked by wind, sediments from floodplain are unimodal distribution, with maximum frequencies about 10% and percentages in GSDs about 90%. In the central bar of the Kangshan River, GSDs of sediments from river channel sands and floodplains fine- grains are constituted by three and five different types of subpopulations respectively. This paper would offer reference information for distinguishing sedimentary environments and researching sedimentary processes quantificationally.