Boiling mechanism of fluid and the possibility of its occurrence in petroleum geology were discussed by studying on synthetic fluid inclusion.The results showed that the occurrences of boiling fluid was mainly in relation to the position of its p-T locus in p-T phase diagram of NaCl-H2O system,where it was located in L V or L V H zone below the TP(H2O)-CP(H2O)-CP(NaCl-H2O)curve and vapor curve of fluid with a certain salinity.Because of the occurrence of deep fault,it was possible that boiling of some oil-gas fluid and NaCl-H2O solution intermittently occurred.This was very beneficial to raising the migration velocity and the maturity of oil-gas fluid and forming natural gas accumulations.