The objective of this study was to investigate the effects of different components of a forest ecosystem on water quality during the rainfall-runoff process. The study area was a small watershed, Huodigou, which is part of Huoditang forest region within the Qinling Mountain Range. For purposes of analysis, the forest ecosystem was divided spatially into four components: canopy, litter, soil, and bedrock. Water samples were collected from rainfall, throughfall, litter drainage, stream water from tributaries within the experimental watershed, and water at the watershed outlet. The samples were analyzed chemically and then compared to determine the effects of different components of the ecosystem on water quality. The analyses indicated that rainfall was slightly acidic, but the pH of the water increased as it moved through each component of the forest ecosystem. The largest increases in the pH were 0.58 units as rainwater passed through the canopy and 0.61 units as water passed through the soil. The NO^-₃, NH⁺₄, K, and PO^3-₄ content of runoff water decreased as it passed through the ecosystem. The largest reduction in NO-3 occurred as water flowed from the tributaries to the watershed outlet. In contrast, the greatest decline in NH⁺₄, K, and PO^3-₄ occurred as water moved through the soil. This is important because the removal of these nutrients from runoff water reduces the probability of eutrophication in the Danjiangkou Reservoir, the main water storage area for the middle line of the South-to-North Water Transfer Project. In general, the Ca and Mg content of water increased as it passed through the ecosystem.The largest Ca increase occurred as water passed through the soil and bedrock, while the main increase in Mg occurred as water passed through bedrock. At the watershed outlet, water contained 0.721μg•L-1 less Cd, 6.528μg•L^-1 less Pb, 0.0128mg•L^-1 less Mn, and 1.4674mg•L^-1 less Zn compared to natural precipitation. Compared to natural rainfall, the throughfall contained 83% less Cd, 77% less Pb, 54% less Mn, and 99% less Zn. Overall, the results from this study show that the canopy has the largest effect and the soil has the second largest effect on water quality in the forest ecosystem.