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　x　AgGaS2　@1.91　mu　m)　among　phase-matching　Sb-based　sulfides,　the　favorable　laser-induced　damage　threshold　(LIDT,　2.3　x　AgGaS2　@2.09　mu　m),　and　the　giant　calculated　birefringence　(0.62@1.91　mu　m).　Structural　analysis　and　computational　simulations　indicate　that　the　highly　polarizable　mixed　anion　determine　the　enormous　SHG　response　and　birefringence.　(c)　2025　Published　by　Elsevier　B.V.　on　behalf　of　Chinese　Chemical　Society　and　Institute　of　Materia　Medica,　Chinese　Academy　of　Medical　Sciences.