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Most butterflies feed on nectar, while some saprophagous butterflies forage on various non-nectar foods. To date, little is known about the genomic and molecular shifts associated with the evolution of the saprophagous feeding strategy. Here, we assembled the high-quality chromosome-level genome of <i>Hestina assimilis</i> to explore its saprophagous molecular and genetic mechanisms. This chromosome-level genome of <i>H</i>. <i>assimilis</i> is 412.82 Mb, with a scaffold N50 of 15.70 Mb. In total, 98.11% of contigs were anchored to 30 chromosomes. Compared with <i>H. assimilis</i> and other Nymphalidae butterflies, the genes of metabolism and detoxification experienced expansions. We annotated 80 cytochrome P450 (CYP) genes in the <i>H. assimilis</i> genome, among which genes belonging to the CYP4 subfamily were significantly expanded (<i>p</i> < 0.01). These P450 genes were unevenly distributed and mainly concentrated on chromosomes 6–9. We identified 33 olfactory receptor (OR), 20 odorant-binding protein (OBP), and six gustatory receptor (GR) genes in the <i>H. assimilis</i> genome, which were fewer than in the nectarivorous <i>Danaus plexippus</i>. A decreased number of OBP, OR, and GR genes implied that <i>H. assimilis</i> should resort less to olfaction and gustation than their nectarivorous counterparts, which need highly specialized olfactory and gustatory functions. Moreover, we found one site under positive selection occurred in residue 996 (phenylalanine) of GR genes exclusive to <i>H. assimilis</i>, which is conservative in most lineages. Our study provides support for the adaptive evolution of feeding habits in butterflies.