Abstract: The present paper dealed with the first intermediate hosts of three species of helminths, Ligula intestinalis, Diphyllobothrium sp. and Contracaecum rudolphii, from the cavity of fish Gymnocypris przewalskii przewalskii(in the Qinghai Lake)which were being fed by the fish host of different size. The results showed that both Ligula intestinalis and Diphyllobothrium sp used Arctodiaptomus salinus and Eucyclops serrulatus as intermediate hosts, but Contracaecum rudolphii only infected E. Serrulatus. Larger fish significantly consumed more A. salinus, but not significantly more E. serrulatus. The population's extinction of L. intestinalis in fish larger than 200 mm was not due to fish's feeding change to the intermediate hosts. The population increases of Diphyllobothrium sp. and C. rudolphii with the growth of fish was related to the increasing composition of larger fish to the intermediate host and the accumulation of worms in the cavity of host.