The detection of Hg2+ ions usually requires large laboratory equipment, which encounters difficulties for rapid field test in most applications. In this paper, we design a reflective sensor for trace Hg2+ analysis based on the fluorescent quenching of Quantum dots, which contains two major modules, i.e. the fluorescent sensing module and the signal processing module. The fluorescence sensing module is composed of a laser source, a light collimated system and a photo-detector, which enables the realization of the fluorescence excitation as well as its detection. The signal processing module realized the further amplification of the detected signal and hereafter the filtering of noises. Furthermore, the Hg2+ concentration will displayed on the QT interface using a Linux embedded system. The sensor system is low cost and small, which makes it available for rapid field test or portable applications. Experimental results show that the sensor has a good linear relationship for the Hg2+ concentration range from 15.0×10-9 to 1.8×10-6 mol·L-1. The regression equation is V0/V=1.309 13+3.37ｃ, where c is Hg2+ concentration, and V0 is the voltage value for the blank case. In our work, the linearity is determined as 0.989 26. The experiments exhibit that Ca2+, Mn2+ and Pb2+ ions have small influence on the Hg2+ detection, and the interfere of other common ions can be neglected, which indicates a good selectivity of the sensor. Finally, it shows that our sensor has a rapid response time of 35 s and a good repeatability, thus it is potential for field test of trace Hg2+.