The concentration of phenol, cyanide, As, Hg, Cr, Cu, Pb, Zn, and F in the Guanting reservoir water has been studied. The analysis shows that the water quality in the reservoir can be divided into three periods, namely 1972–1973, 1974–1978, and 1978–1983. In recent years, the water quality is fairly good. The contents (mg L−1) of the water behind the dam over a long period of monitoring are: phenol 0.001 to 0.009, cyanide 0.001 to 0.002, As 0.001 to 0.04, Cr 0.001 to 0.01, and Hg 0.0005 to 0.002.