In　the　experiment　of　measuring　air-borne　sound　pressure　by　photon　correlation　spectroscopy　in　free　field,　the　accurate　measurement　of　the　angle　between　two　beams　has　a　great　influence　on　the　result　of　sound　pressure　measurement.　By　using　the　magnified　imaging　of　the　microscopic　objective,　the　fringe　spacing　in　the　interference　area　is　measured　and　the　angle　between　the　two　beams　is　calculated　with　high　accuracy.　Firstly,　the　influence　of　the　interference　spot　shape　and　the　uniformity　of　the　fringe　spacing　on　the　sound　pressure　measurement　is　studied　to　optimize　the　optical　configuration.　Then,　the　USAF　test　target　is　employed　as　the　sample　to　calibrate　the　amplification　factor　of　the　system.　Finally,　the　air-borne　sound　pressure　in　free　field　is　measured　by　the　photon　correlation　technology.　The　experimental　results　showed　that,　compared　with　the　angle　obtained　by　traditional　tri-angle　method　by　measuring　the　spatial　propagation　distance　of　light,　the　angle　obtained　by　this　method　improved　the　accuracy　of　air　acoustic　pressure　measurement.　©　2022,　Acta　Metrologica　Sinica　Press.　All　right　reserved.