Pea aphid, Acyrthosiphon pisum, is a serious pest of many different leguminous plants, and it mainly relies on its odorant receptors (Ors) to discriminate among host species.  However, less is known about the role that Ors play in the host plant location.  In this study, we identified a novel conserved odorant receptor clade by phylogenetic analysis, and conducted the functional analysis of ApisOr23 in A. pisum.  The results showed that the homologous Ors from A. pisum, Aphis glycines and Aphis gossypii share 94.28% identity in amino acid sequences.  Moreover, conserved motifs were analyzed using the annotated homologous Or23 from eight aphid species, providing further proof of the high conservation level of the Or23 clade.  According to the tissue expression pattern analysis, ApisOr23 was mainly expressed in the antennae.  Further functional study using a heterologous Xenopus expression system revealed that ApisOr23 was tuned to five plant volatiles, namely trans-2-hexen-1-al, cis-2-hexen-1-ol, 1-heptanol, 4´-ethylacetophenone, and hexyl acetate.  Among them, trans-2-hexen-1-al, which is one of the main volatile organic compounds released from legume plants, activated the highest response of ApisOr23.  Our findings suggest that the conserved Or23 clade in most aphid species might play an important role in host plant detection.