A tandem solar device is the most promising approach for overcoming the single-junction Shockley-Queisser limit, as it can also be manufactured on flexible substrates and is cost-effective and lightweight for a wider range of applications. As is well known, developing high-performance flexible CIGS solar cells is a crucial prerequisite for realizing high-quality tandem devices. To increase the performance of flexible CIGS solar cells, a unique Ga growth profile was created and built in this study, which co-evaporated with Cu and Se in the second step of the well-known “three-step co-evaporation technique.” The best flexible CIGS solar cell with a thickness of around 1.6 μm achieves an impressive efficiency of 18.93 %, not only because a decent double GGI grading can be made, but also the overall grain can be enlarged, which is always difficult to combine well in the past. Finally, an all-flexible 4-terminal (4 T) perovskite/CIGS tandem device was fabricated and demonstrated over 21.5 % efficiency, the highest efficiency of all-flexible perovskite/CIGS tandem solar cells reported in the literature to the best of our knowledge. This study could pave the way for the fabrication of high-quality CIGS thin films and solar devices on flexible polymer substrates.