The invention discloses a device and a method for measuring the free swell ratio of slurry. According to the device and method for measuring the free swell ratio of the slurry, compared with a traditional measuring device (such as measuring cylinder), the device for measuring the free swell ratio of the slurry uses a flexible plastic bag to store the slurry to be measured, along with the swelling of the slurry, the plastic bag would generate deformation, and therefore, the rigid constraint caused by the measuring cylinder is avoided, the slurry suffers less circumferential constraint, the slurry swelling becomes closer to free swelling, and the free swelling of the slurry can be represented more precisely. On this condition, the device for measuring the free swell ratio of the slurry uses a double-cylinder design, wherein the outer cylinder is a rigid cylinder, and the exhaust amount generated by swelling can be obtained precisely through extracting the exhaust amount. Compared with a traditional free swell measuring scheme for the slurry, the device and method for measuring the free swell ratio of the slurry have high representation precision.