The invention discloses a device and a method for determining the expansion rate of tobacco materials by virtue of an air expansion method. The device comprises a sample box, a sample box bracket, a detection tank, a detection tank bracket, a pressure adjusting device and a digital pressure meter, wherein the sample box and the sample box bracket are fixedly arranged in the detection tank; the pressure adjusting device comprises a piston cylinder, a piston rod and a piston rod driving device, the piston rod is matched with the piston cylinder, the piston rod driving device is used for driving the piston rod, and the piston rod is provided with a digital stroke counter; and the digital pressure meter is used for detecting the pressure intensity in the detection tank. The invention provides a novel device for measuring the volume of the tobacco materials. According to the device, the volumes of the tobacco materials before processing and the tobacco materials after processing and expansion can be measured, so that the problem that no national standard and industrial standard on the detection of the expansion rate of the tobacco materials exist is solved; and meanwhile, the device is simple in structure, easy to assemble and maintain, very low in manufacturing cost and easy to popularize.