For　the　influence　of　look-ahead　distance　on　the　smoothness　and　tracking　convergence　rate　of　path　following,　a　line-of-sight　(LOS)　guidance　law　is　designed　with　time-varying　look-ahead　distance　(TLAD).　Firstly,　the　kinematic　position　error　model　of　unmanned　surface　vehicle　(USV)　is　established　with　the　geometric　relation　of　path　following.　Secondly,　the　TLAD　based　LOS　(TLAD-LOS)　is　designed　by　considering　the　time-varying　influence　of　the　cross-track　error　and　surge　velocity.　Thirdly,　a　Lyapunov　function　with　the　position　tracking　error　is　constructed　to　analyze　the　stability　of　the　guidance　system　and　to　design　the　surge　velocity　LOS　guidance　law.　Moreover,　a　heading　controller　is　designed　for　nonlinear　parameter-varying　(NPV)　model　to　achieve　a　fast　and　steady　heading　regulation　of　USV.　Finally,　simulation　results　show　that　TLAD-LOS　exhibits　superior　performance　on　tracking　rapidity　and　stability.　©　2025　IEEE.