The damage of two species of longicorn beetle Asias halodendri(Pallas) and Chlorophorus diadema(Motschulsky) to Tetraena mongolica Maxim was studied at the west Ordos National Nature Reserve in Inner Mongolia  with the approaches of field observation, quadrat method and anatomizing infected plants. The results showed that these 2 species of longicorn beetles had caused severe damage to T. mongolica, which would be the main threat of extinction of T. mongolica. The damage rates were different among 4 sections in the reserve. The distribution, damage characteristics and damage on host varied significantly between the 2 species of longicorn beetles.