Enhancement　of　the　nonlinear　optical　(NLO)　output　power　of　lasers　requires　urgent　development　of　an　NLO　crystal　with　a　significant　second-harmonic　generation　(SHG)　response　and　sufficient　birefringence　for　phase-matching　capability;　however,　simultaneously　optimizing　these　two　key　parameters　remains　a　great　challenge.　In　contrast　to　traditional　single-anion　units,　the　stereochemically-active　lone　pair　Sb3+　ion　is　coordinated　by　S2−　and　I−　ions　to　yield　the　mixed-anionic　SbSI　chalcohalide　that　can　enhance　hyperpolarizability　and　anisotropic　polarizability　concurrently.　As　anticipated,　SbSI　exhibited　the　largest　SHG　response　(5.7　×　AgGaS2@1.91　μm)　among　phase-matching　Sb-based　sulfides,　the　favorable　laser-induced　damage　threshold　(LIDT,　2.3　×　AgGaS2@2.09　μm),　and　the　giant　calculated　birefringence　(0.62@1.91　μm).　Structural　analysis　and　computational　simulations　indicate　that　the　highly　polarizable　mixed　anion　determine　the　enormous　SHG　response　and　birefringence.　©　2025