Abstract: The present application provides a long-wave infrared anti-reflection protective film on a ZnS substrate and a preparation method thereof, wherein the ZnS substrate long-wave infrared anti-reflection protective film comprises a ZnS film layer and a Y2O3 film layer sequentially arranged on the ZnS substrate. The ZnS substrate has a thickness of 2-15 mm, and the ZnS film layer has a thickness of 80-120 nm, the Y2O3 film layer has a thickness of 1000-1300 nm, and a film structure of Y2O3 layer gradually changes from an inside to a surface. The method for preparing a long-wave infrared anti-reflection protective film on a ZnS substrate comprises the following steps: 1) performing a polishing treatment and surface treatment on the ZnS substrate; 2) depositing a ZnS film layer on the ZnS substrate; 3) depositing a Y2O3 film layer on the ZnS film.