Objective To construct lymphocyte activation gene-3 ( Lag-3)-knockout mice and analyze the influence of Lag-3 knockout on mouse phenotypes to provide an animal model for subsequent Lag-3 in vivo function research. Methods Knockout mice were constructed via CRISPR/ Cas9 technology combined with microinjection of fertilized eggs. The Lag-3-knockout mice were screened via molecular identification and further verified via RT-PCR and flow cytometry. Lag-3 gene function in vivo was studied by establishing a ConA-induced liver injury model. Results PCR and product sequencing showed that exon 2-deleted Lag-3-knockout mice were obtained. The heart, spleen, lung and macrophages from ConA-induced homozygous mice were evaluated with only Lag-3 mRNA background expression signals detected in phagocytes and lymph nodes and only very low numbers of Lag-3-positive cells detected in mouse macrophages, spleen cells and lymph nodes. Phenotypic analysis revealed that deleting the Lag-3 gene significantly reduced the number of ConA-induced CD3⁺T cells in the peripheral blood, bone marrow and spleen as well as ConA-induced acute liver injury. Conclusions A Lag-3-knockout mouse model is successfully constructed. In vivo functional studies during liver injury show that Lag-3 deficiency significantly alleviated ConA-induced liver damage.