Abstract: Pachyneuron aphidis was ectoparasites of Aphidius gufiensis. The female adults of Pachyneuron aphidis could mate and oviposite in 24 hours after emergence. The male adults could mate for several times, whereas the females could mate only once. The unmated females could only produce male progeny by parthenogenesis. The adults emerged only in the daytime, with the peak emergence at 7:00～10:00 AM, accounting for 63.6% of the total. Pachyneuron aphidis had preference for various stages of Aphidius gifuensis, with the highest parasitic rate in the old larva, reaching 66.8%, followed by pupa, reaching 29.4%. The adults had the phototropism, upward and thanatosis habit. Feeding on hemolymph of host, aphid honeydew and floral honey, the adults could live longer.