The water chemical elements of Robinia pseud oacacia water protect ion forest in the Miyun reservoir watershed are studied. The results show that the precipitation contained the total chemical elements amount was 12.00 mg/Lin the mainly rainfall period, the highly sample arisen in the June. The consequence of water chemical elements contained as follows: Ca>N> K>Mg>Na>P>Fe>Zn>Cu>Mn. The chemical elements that throug h-fall and stem-flow contained was varied in different direction, but as the sumup of the variance quantity that the total contained amounts were increased as comparison with that of the precipitation. The total chemical elements contained of thr ough-fall, stem-flow was 46.192 m g/ L and 61.586mg/L, respectively, it is 34.191 mg/L and 49.585 mg/L higher than that of prcipitation, respectively.