Abstract: Silver nanoparticles (AgNPs) sol were synthesized by the liquid phase reduction method, with polyvinyl pyrrolidone as the dispersant and trisodium citrate as the reductant. The blue oxTMB was formed after oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) with dissolved oxygen in solution by the catalytic action of AgNPs. The presence of Hg²⁺ could inhibit the catalytic effect of AgNPs, and the color of the above system became lighter. Thus a method monitoring of Hg²⁺ in water was obtained. The proposed method had a good selectivity for Hg²⁺. The optimization conditions found were as follows: reaction time was 2 min, reaction temperature was 20 ℃, amount of 2 g·L^-1 TMB solution was 200 μL, and amount of AgNPs sol was 300 μL. Linearity range of Hg was obtained in the range of 1×10^-7 to 1×10^-6mol·L^-1 with detection limit of 0.01 mg·L^-1. The proposed method was applied to the analysis of water samples, giving values of recovery found by standard addition method in the range of 98.0%-102% and RSDs (n=5) of determined values less than 2.0%.