The widespread destruction of mature forests in China has led to massive ecological degradation, counteracted in recent decades by substantial efforts to promote forest plantations and protect secondary forest ecosystems. The value of the resulting forests for biodiversity conservation is widely unknown, particularly in relation to highly diverse invertebrate taxa that fulfil important ecosystem services. We aimed to address this knowledge gap, establishing the conservation value of secondary forests on Dongling Mountain, North China based on the diversity of geometrid moths - a species-rich family of nocturnal pollinators that also influences plant assemblages through caterpillar herbivory. Results showed that secondary forests harboured geometrid moth assemblages similar in species richness and phylogenetic diversity, but with a species composition distinctly different to assemblages in one of China's last remaining mature temperate forests in the Changbaishan Nature Reserve. Species overlap between these sites was about 30%, and species did not form separate phylogenetic clusters according to site. Species assemblages at Dongling Mountain were strongly differentiated according to forest type; a pattern not found at Changbaishan. Our results indicate that protected naturally regenerated secondary forests in northern China provide suitable habitats for species-rich and genetically diverse geometrid moth assemblages, highlighting the potential importance of these forests for conservation and ecosystem function provision across the wider landscape.