Owing to its high creep rupture strength, good weldability, and low costs, T23 steel is an ideal material for manufacturing the heating components of water walls, superheaters and reheaters in ultra-super critical plants. However, its coarse grain heat affected zone (CGHAZ) is prone to stress-relief cracking (SRC) during post-weld heat treatment or high-temperature service. The mechanism of SRC is controversial and an effective method for forecasting and preventing SRC in T23 components is currently lacking. Clarifying the mechanism of SRC in the CGHAZ of T23 steel, and developing a practical engineering technique for predicting and preventing SRC generation, are therefore essential. In this work, CGHAZ specimens of T23 steel were simulated in a thermo-mechanical simulator, and aged at 650^oC for 0-48 h. After simulating the microstructure evolution of the as-welded CGHAZ during service, the SRC susceptibility of the CGHAZ was evaluated. The microstructural changes and carbide precipitation were observed by OM, SEM, TEM, and EDS. The as-welded CGHAZ of T23 steel was composed of mixed martensite and bainite with high hardness. After ageing at 650^oC, the structure recovered and recrystallized with a lower dislocation density and larger sub-grains than the as-welded CGHAZ. Carbides such as M₂₃C₆, M₇C₃, and MX gradually precipitated inside the grains and grain boundaries, decreasing the hardness. The SRC susceptibility was high in the as-welded CGHAZ, but decreased with increasing ageing time. When the ageing time exceeded 24 h, the sample was SRC-resistant. The main cause of SRC in the CGHAZ was precipitation and growth of M₂₃C₆ on the grain boundaries, which induced the formation of softened zones in the matrix near the grain boundary, and promoted the formation of micro-voids. During ageing, the unstable microstructure in the as-welded CGHAZ transformed as carbides precipitated and the matrix recrystallized, thereby reducing the intragranular strength. Meanwhile, the depletion of alloy elements near the grain boundary was eliminated. The microstructural evolution decreased the difference between the intragranular and intergranular strengths in the CGHAZ. Finally, the CGHAZ showed significantly improved ductility and low SRC susceptibility. The hardness of the aged CGHAZ was positively related to the SRC susceptibility. At hardnesses above 250 HB, the CGHAZ was SRC-susceptible, but at hardnesses below 250 HB, the CGHAZ was SRC-resistant.