We　present　a　method　for　phase　retardation　measurement　of　intracavity　optical　elements　which　is　based　on　frequency　splitting　caused　by　weak　phase　anisotropy　of　Nd:　YAG.　The　measurement　range　covers　0-pi　and　the　measurement　uncertainty　is　less　than　0.0300　rad.　A　theoretical　analysis　is　provided　to　obtain　the　phase　retardation　of　intracavity　optical　elements　by　using　the　phase　difference　or　frequency　difference　of　two　eigenmodes.　The　minimum　error　is　0.0036　rad　by　using　the　composite　wave　plate　to　verify　various　phase　retardation　conditions.　This　work　provides　a　rapid　and　accurate　intracavity　method　for　measuring　the　phase　retardation　of　optical　elements.(c)　2023　Optica　Publishing　Group　under　the　terms　of　the　Optica　Open　Access　Publishing　Agreement