Parasitoid wasps are one of the main natural enemy groups of the spiders. However, the parasitoid wasps of funnel-web spiders are known little so far. Here we studied the parasitoidism of Oxyrrhexis rugosus on the funnel-web spider Allagelena difficilis. Our results showed that mated females of O. rugosus struggled to lay an egg on the dorsal cephalothorax near the thoracic groove of the spider A. difficilis after searching, landing and stinging. The ichneumon larvae consumed nutrition from the host spiders by a particular sucking tube piercing into the spider body. Body color of the larvae turned from light to dark during their developmental duration, which was about 10 days. At the end of the 3rd instar, the larvae left the spider bodies and spun a cocoon in the tunnel of the spider funnel-web. Pupation took place in the cocoon and the adult emerged from the pupa in about 13 days. The adult females and males lived about 11 days. The host spiders were severely affected by the parasitoid O. rugosus. After the ichneumonid larvae hatching, the host spiders won’t prey and molt anymore and became thinner and less active. Till the larvae left their bodies, the host spiders died. Our study reported a new parasitoid of natural enemy of the funnel-web spider and explored detail information about the parasitoid and the host, such as oviposition behavior and developmental characters of the ichneumonid O. rugosus, and the effect of parasitoid on the spider A. difficilis. Our results can benefit further studies on chemical communication and coevolution between the parasitoids and the host spiders.