Hongfeng Lake, a man-made reservoir, is one of the five key drinking-water sources for Guiyang City, the capital of Guizhou Province in southwestern China. In the present research, the distribution and accumulation characteristics of seven heavy metals (Pb, Cd, As, Hg, Cu, Cr and Zn) in the sediment of this lake were analyzed by examining fourteen sediment samples. A method based on toxic-response factor was used to assess the potential ecological risk of these heavy metals to this lake. For comparison, the two sets of reference data representing the pre-industrial and the local baseline pollution levels of interested heavy metals were employed to derive their accumulating coefficients and potential ecological risk indices. The calculated potential ecological risk indices indicate that the lake was polluted by heavy metals, and both mercury and cadmium loadings were critical factors responsible for the ecological hazards posed to Hongfeng Lake among these seven elements.