The invention discloses a diameter measuring instrument which uses wire drawing die members to perform diameter measuring by use of a pneumatic measuring principle. In a gauge head housing, wire drawing dies, pads and compaction blocks are sequentially tightly contacted, and the compaction blocks are connected with a housing through screw threads. The housing is provided with a quick connector, and air subjected to voltage stabilization enters a cavity through the quick connector, flows through the gaps between the wire drawing dies and a measured piece and finally flows out of the interstitial holes of the compaction blocks. The wire drawing dies and the measured piece need to be used in cooperation. During a measuring process, since the change of the diameter of the measured object causes change of the gaps between the wire drawing dies and the measured piece, at the moment, measurement can be carried out by use of a pneumatic measuring apparatus, and conversion into digital signals can be realized. The diameter measuring instrument is mainly applied to measurement of a bare wire diameter.