Epoxy resin/acid anhydride systems are widely used in high-voltage electrical insulation materials, where the formation of a three-dimensional cross-linked network is closely related to volumetric expansion and shrinkage stresses. Therefore, investigating the gelation and glass transition behavior as well as stress evolution during curing at the microscale is of great significance. Under isothermal curing conditions at 120 ℃, this study employed a combination of atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and stress monitoring to characterize samples at various curing stages. Results indicate that, during the early stage of curing, esterification between acid anhydride and epoxy groups promotes chain growth and increases molecular weight. In the gelation stage—the middle curing phase—the reaction rate reaches its maximum, leading to the formation of heterogeneous chemical clusters and a significant enhancement in chemical shrinkage, with a peak strain of 706 με observed. During the later curing stage, a stable three-dimensional network gradually forms, and the internal stress stabilizes upon transitioning into the glassy state. This study reveals the kinetic transition from chemically controlled to diffusion-controlled curing, providing a theoretical basis for understanding crosslinking behavior and optimizing the curing process of electrical insulating materials.