The invention discloses a device and a method for detecting critical flow velocity and settlement rate of backfill slurry, and belongs to the field of mining backfilling. A feeding pipe of the detection device is fixedly mounted on an upper part of a cylindrical barrel and is communicated with the cylindrical barrel; a laser speedometer and a scale are respectively fixed on an outer part of the cylindrical barrel; a straight angle stirring rod and a variable-frequency speed regulation motor are arranged in the cylindrical barrel in a connecting manner and are coaxial to the cylindrical barrel; a high-definition camera is supported by a hydraulic support and positioned outside the cylindrical barrel; through circumferential motion of the backfill slurry in the cylindrical barrel, the flow velocity of the slurry is detected by using the laser speedometer, and a settlement process of the backfill slurry is observed by using the high-definition camera, so that the critical flow velocity and the dynamic settlement rate of the backfill slurry in a flowing process can be obtained. According to a detection result, through theoretical analysis, a critical flow velocity mathematical model, and a dynamic settlement rate mathematical model of the backfill slurry and a correlation are established. According to the detection device and method disclosed by the invention, a certain and reliable scientific support is provided for theory and technical study on pipeline transportation of the backfill slurry.