Since　the　sideslip　angle　is　often　ignored　or　simplified　in　the　process　of　path　following　of　unmanned　surface　vehicle　by　using　the　line-of-sight　(LOS)　guidance　law　because　of　its　fast　change　and　the　difficulty　of　measurement,　an　observer　was　proposed　by　internal　model　control　(IMC)　to　quickly　estimate　the　sideslip　angle　in　the　LOS　guidance　law.　First,　a　prediction　model　was　established　for　the　tracking　error,　and　a　state　space　model　for　prediction　errors　was　constructed　as　an　internal　model.　With　the　introduction　of　the　auxiliary　variables,　a　new　augmented　system　was　set　up　for　a　state　space　model　of　the　prediction　errors.　Then,　the　sideslip　angle　observer　was　designed　by　the　theory　of　state　feedback　with　the　feature　of　the　control　law　of　a　proportional-integral　type.　Theoretically,　the　stability　of　the　system　was　proved　based　on　the　Lyapunov　criteria.　A　simulation　and　experiment　verified　the　effectiveness　of　the　proposed　sideslip　angle　observer　in　improving　the　path-following　accuracy.　The　results　show　that　the　IMC-based　observer　introduces　a　proportional　term　of　tracking　error　that　is　not　considered　by　other　observers,　which　is　easier　to　implement　and　adjust,　and　has　a　faster　response　speed　and　a　smaller　steady-state　error　for　the　sideslip　angle.　In　addition,　the　assumption　of　a　small　sideslip　angle　is　not　introduced　in　the　design　process,　so　the　proposed　observer　provides　an　accurate　estimation　method　for　a　large　sideslip　angle.