Heavy-metal pollution of groundwater is a severe threat to the environmental safety and human health. Owing to their high sensitivity and good selectivity, fluorescence probe detection methods have been commonly used to determine the concentration of heavy metals. Because of the large size and high cost of the light source and optical system, the current heavy-metal fluorescence detection devices cannot meet the demands of in situ detection. In this study, we develop an in situ fluorescence detection device for the analysis of heavy metals in groundwater samples. The optical detector of the device uses ultraviolet (UV) light-emitting diodes (LEDs) as the light source and exhibits a confocal optical path design with a fluorescence collection angle range and an outer diameter of up to 102° and 60 mm, respectively, to achieve miniaturization and decrease cost. A commercial benchtop fluorescence spectrophotometer was used as a reference device. The devices were equipped with identical Hg²⁺ ion fluorescent probes, and comparative performance tests were conducted. The experimental results demonstrate that the as-developed fluorescence detection device exhibits good stability and a remarkable linear response, with a correlation coefficient (R²) and detection limit of 0.989 and 1.47×10^-9, respectively. Furthermore, the performance parameters of the fluorescence detection device are comparable to those of the benchtop fluorescence spectrophotometer.