In this study, a stacked high-Ge-concentration SiGe/Ge multilayer on a novel Ge + SiGe strain-relaxed buffer (SRB) structure and its thermal stability for gate-all-around (GAA) MOSFETS is systematically investigated. First, a novel SiGe capping of Ge SRB structure is introduced to realize a high quality of stacked high-Ge-concentration SiGe/Ge multilayer. This novel Ge + SiGe SRB has successfully suppressed the threading defects at the bottom of the Ge SRB and the top region of the Ge and SiGe SRB are defect free. Meanwhile, Ge concentration of SiGe SRB is stepped down from 85 to 75% and its thickness is about 150 nm. As a result, SiGe SRB can be maintained well during the following channel release process if the high-Ge-concentration SiGe is selected as channel. In addition, to verify the thermal stability of the stacked high-Ge-concentration SiGe/Ge multilayer, rapid thermal anneal (RTA) treatment is performed at different temperatures in N₂ ambient. It is found that the Ge concentration and thickness of high-Ge-concentration SiGe of sample treated by RTA at 600 °C is almost the same as the as-grown sample. However, as the temperature of RTA treatment of samples raises to 700 °C and above, obvious interdiffusion of Ge and Si elements can be observed in the SiGe/Ge multilayer. Therefore, the highest tolerable temperature of 600 °C is verified for the thermal stability of the high-Ge-concentration SiGe/Ge multilayer.