The　roll　angle　measurement　method　based　on　a　heterodyne　interferometer　is　an　efficient　technique　for　its　high　precision　and　environmental　noise　immunity.　The　optical　layout　bases　on　a　polarization-assisted　conversion　of　the　roll　angle　into　an　optical　phase　shift,　read　by　a　beam　passing　through　the　objective　plate　actuated　by　the　roll　rotation.　The　measurement　sensitivity　or　the　gain　coefficient　G　is　calibrated　before.　However,　a　relative　tilt　between　the　laser　and　objective　plate　always　exist　due　to　the　tilt　of　the　laser　and　the　roll　of　the　guide　in　the　field　long　rail　measurement.　The　relative　tilt　affect　the　value　of　G,　thus　result　in　the　roll　angle　measurement　error.　In　this　paper,　a　method　for　field　calibration　of　G　is　presented　to　eliminate　the　measurement　error　above.　The　field　calibration　layout　turns　the　roll　angle　into　an　optical　path　change　(OPC)　by　a　rotary　table.　Thus,　the　roll　angle　can　be　obtained　from　the　OPC　read　by　a　two-frequency　interferometer.　Together　with　the　phase　shift,　an　accurate　G　in　field　measurement　can　be　obtained　and　the　measurement　error　can　be　corrected.　The　optical　system　of　the　field　calibration　method　is　set　up　and　the　experiment　results　are　given.　Contrasted　with　the　Renishaw　XL-80　for　calibration,　the　proposed　field　calibration　method　can　obtain　the　accurate　G　in　the　field　rail　roll　angle　measurement.