The interaction between plants and phytophagous insects is one of the most complex relationships in ecosystems. By acting as direct third-party participants, gut symbionts redefine this binary antagonistic relationship. This article reviews the roles of gut symbionts in the adaptive evolution of phytophagous insects, highlighting their important roles in degrading plant secondary metabolites, modulating plant defense responses, promoting insect nutrient absorption, and shaping immune phenotypes. Gut symbionts not only enhance the adaptability of insects by degrading plant defense compounds, but also significantly influence their physiological adaptation by manipulating plant defense signaling pathways, regulating the immune system of insects, and promoting their rapid adaptation to external stress. When insects are confronted with environmental changes or shifts of host plants, the dynamic plasticity of the gut symbionts provides them with evolutionary advantages. Reviewing the mechanism of action of intestinal symbiotic bacteria in the adaptive evolution of insects is helpful to deepen our understanding of the ecological interaction process between insects and plants.