With net sack and coring methods, this paper analysed the seasonal dynamics of litter and under-ground biomass of Carex lasiocarpa population in Sanjiang Plain wetland. The standing litter showed exponential seasonal dynamics. The decomposition rate of litter was indicated by the rate of its weight loss, and the daily weight loss was gradually decreased from 0.7058% to 0.2372%. Carex lasiocarpa population produced 210.8876 g.m-2 litter in its vegetative season (from May 2, to October 10, 1999), and its under-ground biomass had an obvious vertical structure, like a converse pyramid or a parabola.