Infrared nonlinear optical (IR NLO) materials are significant in laser technology for civil and military uses. Here, we report the synthesis, structural chemistry and NLO properties of a halogen‐rich chalcohalide Sn7Br10S2. Its noncentrosymmetric (NCS, P63) structure can be considered as partially aliovalent anion substitution of SnBr2 (P63/m) induced centrosymmetric (CS) to NCS structural transformation. The 3D ∞Sn(1)6Sn(2)6Br6X66− (X=Br/S) channel framework consists of Sn(1)BrX2 and Sn(2)X3 trigonal pyramids. It exhibits excellent NLO performance, including a strong phase‐matchable NLO response of 1.5 × AgGaS2and high laser‐induced damage threshold of 6.3 × AgGaS2.Investigation of the structure–NLO performance relationship confirms that the effective arrangement of Sn(1)BrX2 and Sn(2)X3 units predominantly contributes to the large SHG response. These results indicate Sn7Br10S2 is a potential IR NLO candidate and provides a new feasible system for promising NLO materials. The first ternary halogen‐rich nonlinear optical chalcohalide Sn7Br10S2 exhibits a strong phase‐matchable second‐harmonic generation response (1.5 × AGS@2.1 μm) and high laser‐induced damage threshold (6.3 × AGS). This work provides a competitive candidate with good performance, facile synthesis and simple chemical composition for infrared nonlinear optical (IR NLO) applications, and also introduces a facile strategy to obtain high‐performance NLO materials via a symmetry break.