In this paper, the Pinus koraiensis plantation at mountainous region in Heilongjiang Province is the research object. Four different types of water in Pinus koraiensis plantation including rainfall, throughfall, stemflow and runoff are respectively selected to study the influence of Pinus koraiensis plantation on the water quality from May to November of 2004. The indicator included pH, Do, TURB, TDS, COND, ORP, K, Na, Ca, Mg, Cu, Zn, Mn and Fe. The results show that the chemical properties of rainfall have been changed after it passing through forest canopy. The value of pH, Do and ORP in throughfall are lower than those of rainfall, the value of TURB and TDS increase obviously, 3.2 and 6.4 times higher than the rainfall. The concentrations of Na, Ca and Zn in the throughfall are lower than those in the rainfall but other metal elements show higher concentrations. The value of pH and Do in stemflow are lower than other water. The element contents in stemflow increase greatly except Cu. The values of pH, TURB, TDS and COND in the runoff are higher than those in stemflow, throughfall and rainfall. The variance of pH and Do don’t change significantly, and the variance coefficients of pH and Do are lower in rainfall, throughfall, stemflow and runoff. The value of TURB, TDS and COND vary remarkably, and the variance coefficients of which are higher. The leaching coefficient of throughfall is in the following order: K>Mg>Mn>Fe>Cu>Na>Zn>Ca. The leaching coefficient of stemflow is in the order of Fe>Mn>Mg>K>Na>Ca>Cu>Zn. After atmospheric precipitation passing through the Pinus koraiensis plantation, the value of pH in the throughfall, stemflow and runoff increase gradually closer to neutral, which shows that the Pinus koraiensis plantation have good buffer function to the acidification of the water.