Radiation-free　photonic　bound　states　in　the　continuum　(BIC)　in　metasurfaces　allow　ultrahigh　quality　(Q)　factor　and　strongly　confined　mode　volume,　which　are　extremely　advantageous　in　the　development　of　ultrasensitive　microcavity　sensors.　However,　the　conventional　isolated　BICs　are　susceptible　to　failure　due　to　symmetry　breaking　caused　by　fabrication　imperfection　and　nonzero　incident　angle.　Here,　we　propose　a　silicon　nitride-based　metasurface　with　multiple　BIC　merging.　The　merging　of　accidental　BIC　and　symmetry-protected　BIC　can　increase　the　Q-factor　near　the　Brillouin　zone　Γ　point　and　thus　robustly　induces　a　figure　of　merit　(FOM)　of　refractive　index　sensing　at　small　incident　angles　two　orders　of　magnitude　higher　than　that　in　isolated　BIC　configuration.　Specifically,　the　FOM　in　merging　BIC　reaches　108　at　a　2°　incident　angle.　The　BIC　merging　can　be　universally　achieved　in　square　lattices　with　C4　symmetry,　and　slower　decay　of　Q-factor　and　higher　FOM　can　further　occur　in　hexagonal　lattices　benefiting　from　higher-order　topological　charges.　The　advantage　of　merging　BIC　is　also　maintained　when　considering　in-plane　and　out-of-plane　symmetry　breaking.　These　results　offer　a　unique　design　path　for　high-performance　metasurface　sensors　and　can　be　extended　to　other　high-Q　applications　such　as　low-threshold　lasers,　nonlinear　frequency　conversion,　and　low-loss　waveguides.　©　2024　Optica　Publishing　Group.　All　rights.