We report an ultrasensitive electrochemical sensor for Hg2+ detection, on the basis of two Hg2+-specific oligonucleotide probes (a thiolated capture probe and a biotinated signal probe), "terminal" signal amplification of alkaline phosphatase catalyzed deposition of silver and in situ microliter-droplet anodic stripping voltammetry (ASV). The thiolated capture probe is immobilized on an Au-plated glassy carbon electrode, and the biotinated signal probe is then attached to the electrode surface through the thymine-Hg2+-thymine interaction in the presence of Hg2+. A streptavidin-alkaline phosphatase (ALP) composite is then immobilized on the electrode surface via biotin-streptavidin interaction. The immobilized ALP can catalyze the hydrolyzation of ascorbic acid 2-phosphate trisodium salt to generate ascorbic acid, which can reduce AgNO3 to form silver deposit on the modified electrode. Quantitative analysis of Hg2+ is conducted through microliter-droplet ASV of silver after its simultaneous chemical dissolution and cathodic preconcentration on the modified electrode. The sensor can linearly respond to the common logarithm of Hg2+ concentration from 0.1nM to 250μM with a limit of detection of 0.01nM (2 ppt, S/N = 3) and a sensitivity as high as 227μAdec-1.