Interaction between the zincon (zinc reagent) and trace mercury in pH 6.70 buffer solution (the system KH2PO4_NaOH) gives rise to a purplish red complex compound. There is a linear relation between the absorptivity and the Hg2+content at the maximum absorption. The peak of the absorption spectrum isλmax=545 nm, which has a bathochromic shift of about 55 nm relative to zincon (the peak of zincon isλmax=490 nm). The apparent molar absorptivity is 2.8×105L·mol-1·cm-1. The standard curve is in the range of 0.1~25μg·mL-1, obeying the Lamber_Beer law, and the detection limit is 0.01μg·mL-1for Hg2+at same conditions, basic conditions of determination were studied. The reaction is fast, senstive and easy to operate and shows a good reproducibility. It can be applied in the determination of trace mercury in industrial waster water, soil, minerals and organisms.