To meet the requirements for the rapid, on-site and high-sensitivity detection of mercury ions (Hg 2+) existing as the environmental heavy metal pollutant, we have independently developed an all-fiber sensor for Hg 2+ detection based on the fluorescent quenching of quantum dots and optical fiber evanescent wave sensing. It is mainly composed of an optical fiber probe module, an optical module and a signal processing module, which realizes the excitation and collection of fluorescent signal and the processing and acquisition of photoelectric signal. The experimental results show that the detection limit of the sensor for Hg 2+ reaches 1 nmol/L. Moreover, the fluorescent quenching rate of quantum dots varies linearly with the logarithm of the Hg 2+ concentration in the range of 1 nmol/L to 500 nmol/L, and the linear correlation coefficient is 0.9867. Meanwhile, the ion anti-interference experiment confirms that the sensor has good selective recognition for Hg 2+ detection. The sensor is also applied to the environmental water sample detection with the recovery of 90.1%-97.3%. The sensor system not only has high sensitivity and fast response speed, but also can realize remote detection and real-time monitoring. It is also conducive to the integration and miniaturization of the instrument. Therefore, it has a broad application prospect in the detection of heavy metal ion pollution.