Previous studies have indicated that forests have the ability to purify rainwater; rainwater gets filtered as it passes through various forest layers (such as canopy and litter) before entering the surface water, and this filtered rainwater get further purified by the soil layers during its transportation to the ground water. This combination of the forest and soil layers usually contribute to the maximum hydrological functions of forest ecosystems. Heavy metals in the atmosphere combine with rainwater and enter the surface water; this water is intercepted by the canopy, litter, and soil during precipitation, and consequently the rainwater is purified. The southwest region including Chongqing, Sichuan, Guizhou, Yunnan and Xizang is the second largest district in China with a high forest coverage rate (95.41%); the annual rainfall here ranges from 900 mm to 1522 mm. The Luosi Mountain (altitude: 1440m) in Simian Mountain is a representative area of the southwest region with high forest coverage (canopy density: 0.91); this is an appropriate site to study the purification of heavy metals by forests. Two common harmful heavy metals, lead (Pb) and cadmium (Cd), mainly released from industrial waste gases and automobile exhaust, were chosen to analyze their dynamic characteristics in the evergreen broad-leaved forest, a typical subtropical forest ecosystem in Simian Mountain. In addition, the interception abilities of different ecosystems for Pb and Cd were also compared. Precipitation, forest precipitation (throughfall and dropping off), litterfall, and soil leachate of the evergreen broad-leaved forest in Simian Mountain were investigated from September 2012 to August 2013, with sampling being conducted twice a month. The results showed that: 1) during the study period, the concentrations of Pb (22.92±13.57)μg/L and Cd (2.82±1.46)μg/L in the precipitation were respectively, and the monthly average deposition fluxes of Pb and Cd were (3.06±2.74) mg/m² and (0.32±0.27) mg/m² higher, respectively than those in other regions, such as Tangshan in China, New Jersey in USA, and North Sea in Belgium. Rain water in this forest was to some extent polluted by Pb, with the concentration of Pb in the precipitation meeting the third water quality standard of the Quality Criteria of Ground Surface Water Environment (GB3838-88), but it was not polluted by Cd. However, the concentrations of Pb (2.44±1.77)μg/L and Cd (0.13±0.04)μg/L in the leachate reached the first water quality standard after being intercepted by different levels of the evergreen broad-leaf forest, suggesting that 1) the interception by various elements during precipitation considered in our study facilitates purification of rain water; 2) the atmospheric precipitation was the main source of Pb and Cd in this forest region; 3) the total interception rates of Pb and Cd by the forest ecosystem were up to 98.52% and 99.09%, respectively, and the canopy layer showed the strongest interception ability among the various interception factors during precipitation in this ecosystem, with interception rates of 91.67% and 86.84% for Pb and Cd, respectively followed by the litter layer, with rates of 67.86% and 79.74%, respectively. 4) The concentration of Pb in leachate was higher than that in litterfall, indicating that soil could release Pb into leachate. However, the soil showed an opposite function for Cd, as Cd levels in leachate were lower than that in litterfall, probably suggesting that the soil can adsorb Cd.