Abstract: A gas leak detection device and a detecting method are provided. The gas leak detection device includes a remote temperature sensor and a processor. The remote temperature sensors detect an area-to-be-monitored to obtain a plurality of thermal images. Each pixel position in the thermal images includes a corresponding temperature value. The processor obtains the thermal images from the remote temperature sensor, calculates a detected temperature value of each environmental block according to each of a plurality of environmental blocks in the thermal images, calculates a temperature judgment threshold based on the detected temperature value of each environmental block in a detection time period, determines whether a temperature of at least one region-of-interest is abnormal based on the temperature judgment threshold, and performs a warning operation when the temperature of the at least one region-of-interest is abnormal.