GH3230 superalloy has excellent high-temperature mechanical properties and oxidation resistance; hence, it is widely used to manufacture key high-temperature structural components in the aerospace and other fields. In the research, development and production process of aero-engines and gas turbines, GH3230 superalloy plate is often used to make the welded components of a combustion chamber. Excellent weldability is an important technical index and is also an important basis for the plate component design and welding process formulation. For GH3230 superalloy welded plates used at high temperature, the stability of the microstructure of the welded joint is closely related to the mechanical properties of welded components. However, in theory, some deficits remain in the in-depth study of scientific problems regarding GH3230 superalloy welding; hence, the microstructure evolution of welded joint during its long-term service at high temperature has attracted attention. This study focuses on observing and analyzing the experimental phenomena of microstructural changes in welded joint (in the weld and heat affected zones), changes in the alloy element content of carbides, the coarsening rate of carbides, and degradation reaction between carbides after long-term thermal exposure at various high temperatures. Experimental results show that in the tungsten argon-arc welded thin plate of GH3230 superalloy, the dendrite morphology of the weld zone disappears after 2000 h of thermal exposure. Furthermore, the degree of dendrite segregation decreases remarkably, grain size in the heat affected zone becomes uneven, and carbide precipitation in the grain and grain boundary increases obviously. After long-term thermal exposure at various temperatures, the solid solubility and content of alloying elements in two types of carbides—(Cr, W)₂₃C₆ and (W, Cr)₆C—in the welded joint change and coarsening rate of M₂₃C₆ carbide exhibits an obvious increase. During the long-term thermal exposure of the argon-arc welded thin plate, the microstructures of the weld and heat affected zones undergo alloy carbide degradation based on diffusion, which changes the type, quantity, size, morphology, and location distribution of carbide.