A　real-time　displacement　reconstruction　technique　based　on　a　phase　retardation　method　is　proposed.　By　introducing　a　waveplate　and　a　polarization　beam　splitter　into　the　external　cavity,　two　self-mixing　signals　with　a　phase　shift　of　pi/2　can　be　obtained.　Then　the　displacement　of　the　external　object　can　be　reconstructed　by　a　quadrature　demodulation　algorithm,　which　significantly　simplifies　the　data　processing　and　meets　the　requirements　of　real-time　displacement　reconstruction　techniques.　The　results　show　that　the　absolute　errors　of　the　reconstructed　displacements　are　less　than　75　nm.