Background: The Niemann–Pick C2 (NPC2) gene family plays an important role in olfactory communication, immune response, and the recognition of host plants associated with environmental adaptation for insects. Methods: In this study, we conducted a genomic analysis of the structural characteristics and physicochemical properties of the NPC2 genes of eleven butterfly species available, focusing on the alpine Parnassius species, especially Parnassius glacialis, to investigate their duplication and expression patterns. Results: Our results indicate that a significant expansion of NPC2 genes was detected in P. glacialis compared to other butterflies; in addition, the expansion of these unevenly distributed P. glacialis NPC2 chromosome genes was shaped by tandem duplication mediated by transposons. Furthermore, the PgNPC2 genes had relatively higher expression in P. glacialis antennae and other head tissues. These facts were verified by quantitative real-time PCR (qRT-PCR). Conclusions: These findings suggest that the expansion of NPC2 genes may have contributed to the local adaptation of P. glacialis during its dispersal ‘out of the Qinghai–Tibet Plateau’, although further functional tests are needed to confirm their specific role in this adaptive process.