Cylindrical　roller　bearings　are　widely　used　in　various　types　of　large　equipment　under　low　speed　and　heavy　load　conditions　due　to　their　linear　contact　characteristics.　Their　operating　performance　and　stability　are　closely　related　to　the　contact　lubrication　state　between　rollers　and　the　inner　or　outer　rings.　The　ultrasonic　method　based　on　equivalent　stiffness　can　be　used　to　measure　the　thickness　of　elastohydrodynamic　lubrication　film　of　roller　bearings　under　actual　working　conditions.But　it　cannot　be　directly　applied　to　the　measurement　of　oil　film　thickness　in　mixed　lubrication　state　under　low　speed　and　heavy　load　conditions.　Therefore,　an　ultrasonic　measurement　method　under　mixed　lubrication　was　proposed　in　this　paper,　and　the　parallel　model　of　interface　oil　film　stiffness　and　rough　body　contact　stiffness　was　established.　By　introducing　the　contact　coefficient　and　combining　with　the　empirical　formula,　the　total　interface　stiffness　measured　by　ultrasonic　method　was　decomposed　to　get　the　oil　film　stiffness　under　mixed　lubrication　state.Then　a　more　accurate　oil　film　thickness　was　obtained.　Finally,　through　comparing　the　experimental　and　theoretical　results,　the　feasibility　and　validity　of　the　proposed　model　were　verified.　©　2020,　Editorial　Office　of　Journal　of　Vibration　and　Shock.　All　right　reserved.