Insect sex pheromones have been widely used in integrated pest control due to their efficiency, non-toxicity, specificity, and environmental sustainability. They are considered a key component of green pest management techniques. Polymerus pekinensis is a phytophagous plant bug on alfalfa (Medicago sativa L.) in East Asia. This study used gas chromatography–electroantennogram detection (GC–EAD) and gas chromatography–mass spectrometry (GC–MS) to analyze the whole-body extracts from male and female P. pekinensis. Octyl acetate (OA) and decyl acetate (DA) elicited the antennal response of males and were identified as the predominant components of female and male extracts, respectively. Subsequent field trials demonstrated that OA (>8 mg per lure) showed the strongest attraction to conspecific males. However, when DA was added in a lure (≥2 mg), a significant decline in captures occurred. These findings provide new insights into the understanding of sex pheromones in Miridae and benefit the development of sustainable management of P. pekinensis.