The utility model provides a volume measuring device for an inner cavity of a cavity. The device is mainly composed of a communicating cavity, an air pressure standard cavity, a differential pressuregauge, a micrometer caliper and the like. An electromagnetic valve for controlling the communication cavity and the measured cavity to be communicated and separated is arranged in the middle of the adapter tube; the micrometer caliper mounting base is connected with the communicating cavity and is internally provided with a dynamic sealing structure; the first pressure leading interface is connected with a positive end pressure measuring port of the differential pressure gauge through a first pressure leading pipe; the second pressure leading interface is connected with a negative end pressuremeasuring port of the differential pressure gauge through a second pressure leading pipe; measuring the differential pressure between the communication cavity and the air pressure standard cavity through a differential pressure gauge; a screw of the micrometer caliper smoothly enters and exits from the communicating cavity through the dynamic sealing structure, and the size of the communicating cavity can be changed. The measurement result can be metered and traced, the measurement device is simple and reliable, the measurement method is simple and easy to implement, and the measurement result accuracy is high.