A quick measuring method used to measure the case volume by air is established because when it is measured by water, the operation is complicated, the time to dry the case is long and it goes against with the automatic production. This paper explains the working principle and realization mode for measuring the case volume by air, and analyzes the realization mode systematically and analyzes the measuring range of the measuring system with certain volume. After analysis, piston cylinder with certain volume can be used to quickly measure the volume within certain range while ensuring the relative measuring error (or measuring accuracy) of the case volume.