Because of precipitation serves as one of major approaches of nutrient input into the forest ecosystem, the accurate measurement of its volume and ion concentration is of prime importance in an evaluation of nutrient biogeochemical cycle. Therefore, the chemistry of precipitation within a year was monitored and analyzed in a primary forest in the Maolan Natural Reserve, a Karst region in Libo, Guizhou Province. The movement patters of nutrient elements through hydrological processes could be understood by monitoring chemistry data of water. The investigation period was from September, 2007 to August, 2008. The results showed: (1) The pH and most concentrations of most nutrient elements in precipitation, throughfall, stemfall and streamflow varied seasonally except for Na+, with maxima in winter or early spring and minima in summer. This may be resulted from the concentration dilutions or inspissations due to the varieties of water discharge. (2) The nutrient elements in throughfall and stemflow were significantly higher than those in the precipitation when rainfall passed canopy. The concentrations of K+, Mg2+, and Ca2+ were altered more than the other nutrient elements. The concentration of K+ in throughfall and stemflow was 14 times and 21 times higher than those in precipitation, respectively. The concentration of Mg2+ in throughfall and stemflow was 12 times and 9 times higher than those in precipitation, respectively. The concentration of Ca2+ in throughfall and stemflow was 4 times and 3 times higher than those in precipitation, respectively. This probably resulted from the exchange of cations and the canopy leaching. (3) Through throughflow and stemflow, the most nutrient elements which was input the sites were K+ and Ca2+ i.e. 35.8 and 31.5 kg hm-2 a-1, respectively. The most nutrient elements output through streamflow were Ca2+ and Mg2+, i.e. 579.5 and 1529 kg hm-2 a-1, respectively, SO2-4-S coming next, Na++K++NH+4-N accounts for less than 1% of the total cations, and Cl-+NO-3-N accounts for less than 1% of the total anions. These general features indicated that the chemical composition of water was largely controlled by carbonate weathering. (4)The characteristics of annual vertical movements of elements indicated the concentrations of nutrient elements in soil water were varied from 0cm to 5cm. The concentration levels of NH+4-N and NO-3-N in the soil water suggested that the transformation might be caused by microbes through nitrogen fixation, nitrification and uptake by vegetation. (5) The nutrient budget (e.g. throughfall+stemflow-streamflow) within one year in the forest was 2.9 for N, 25.2 for K, -547.4 for Ca, -144.5 for Mg, -4.0 for Na, 2.5 for Cl, -5.9kg hm-2 a-1 for SO2-4-S, respectively. Our results showed that annual inputs exceeded outputs for dissolved inorganic N and K+, The watershed had net losses of Ca2+, Mg2+, and was close to steady state for Na+ and Cl- in the forest.