To　reduce　or　restrain　the　nonlinearity　in　laser　heterodyne　interferometer　via　adjusting　optical　component,　the　mechanism　and　the　method　of　measuring　nonlinearity　need　to　be　researched.　Based　on　the　spectrum　analysis　of　the　measurement　signal,　a　method　to　measure　the　nonlinearity　is　proposed.　With　the　model　of　the　amplitude　relationship　between　the　nonlinearity　and　measurement　signal,　the　first　harmonic　nonlinearity　and　the　second　harmonic　nonlinearity　are　counted.　Then,　the　nonlinearity　is　measured　accurately.　The　experimental　result　shows　that　the　method　can　measure　the　nonlinearity　accurately　and　can　easily　be　adopted.