Abstract: Insect-borne diseases are a class of major forest diseases that occur globally in epidemic proportions. Both insect pests and pathogens form synergetic infection to forest trees, which are extremely difficult to control due to the complexity of their mechanisms of occurrence, the insidious nature of damage symptoms, and mutualistic symbioses between insect pests and pathogens, which increase the fitness of each individual species, enhancing the buffering and adaptive capacity of species to environmental change, and maintaining specific ecological functions. Bark beetles (Curculionidae Scolytinae) and fungi have formed stable ectosymbiosis during the long-term co-evolution process. These fungi are called associated fungi of bark beetles, many of which can cause serious forest diseases and show close mutualistic symbioses with beetles. The present paper focuses primarily on the bark beetle-ophiostomatoid fungi companion system, which is the most well understood associated fungi, and provides an overview of the latest research progress in related fields from three perspectives, namely, the diversity of associated fungi, ecological functions of ophiostomatoid fungi, and prospect of strategies for prevention and control of the synergetic infection of insect pests and pathogens. It is hoped that the findings summarized in this review can aid in building a scientific basis for understanding the mechanisms of action of such damage and offer novel ideas for effective control of forest insect pests and pathogens.