The establishment of nature reserves is recognized as one of the most effective approaches for the in-situ conservation of biodiversity. In this study, based on the data from basic investigations of nature reserves all over the country, we analyzed populations of wild mammals and their distribution in the nature reserves of China and evaluated the in-situ conservation status of 591 species in nature reserves on a national scale. According to the evaluation index system, we divided the in-situ conservation of wild mammals in nature reserves of China into seven levels, namely "Effectively Protected", "Well Protected", "Moderately Protected", "Less Protected", "Status Unknown", "Not Protected", and "Not Evaluated." Results showed that, among the 591 species of wild mammals, 556 were evaluated and 35 were not. We found that 534 species were protected to different extents, accounting for 90.36% of the 591 species of wild mammals in this study. Among these 534 species, 145 were "Effectively Protected", 60 were "Well Protected", 100 were "Moderately Protected", and 229 were "Less Protected", accounting for 24.53%, 10.15%, 16.92% and 38.75%, respectively, of the 591 species of wild mammals in this study. However, 22 species were classified under "Status Unknown." For the different protection systems, the in-situ conservation status of wild mammals were evaluated, including the 140 and 427 species listed under the national key protection and China Species Red List, respectively, of which 109 and 409 were protected to varying degrees. Although most of the wild mammals had been put under in-situ conservation in nature reserves of China, and most of the key protection wild mammals were well protected, their protection status was still not optimal and their conservation needed to be further intensified as the percentage of efficient protection of wild mammals, including "Effectively Protected" and "Well Protected", was still significantly lower. Regarding species with unknown protection levels, it is necessary to carry out more comprehensive and detailed scientific investigations to determine further their conservation status in the nature reserves. For the species of wild mammals that are unprotected by nature reserves, it is essential to include them in the protection system by improving the network of nature reserves in China to fill the gaps.