Free-standing polymer films that are ultra-thin, lightweight, and robust hold significant promise for applications in flexible electronics. However, their performance has been constrained by the challenge of simultaneously enhancing mechanical strength and reducing density. Herein, an ultra-thin and robust polyimide (PI)-based covalent organic framework-modified multi-walled carbon nanotube (MWCNT-COF) film is demonstrated for high-specific-power flexible GaAs solar cells. The film exhibits a high Young’s modulus of 1.53 GPa, a tensile strength exceeding 97.53 MPa, and a low density of 0.73 g cm−3, representing a 45.1% reduction in density. The incorporation of 3D interconnected layered and porous MWCNT-COF significantly enhances the film’s mechanical properties while reducing its density. The synergistic effects of robust MWCNTs and covalently bonded COF layers, combined with the thermally stable PI molecular structure, impart exceptional strength and lightness to the film. Computational studies further confirm that COF-grafted MWCNTs effectively modify the polyamide acid matrix, substantially improving the mechanical properties of the PI films. Moreover, flexible dual-junction and triple-junction GaAs solar cells were successfully fabricated using 6 µm-thick MWCNT-COF/PI composite films, delivering outstanding specific powers of 8998 and 7749 W kg−1, along with efficiencies of 26.6% and 31.5% (AM0), respectively. These results underscore the potential for high-performance applications in space and energy systems. This work offers valuable insights into the development of high-performance flexible GaAs solar cells based on ultra-thin and robust PI films for a wide range of applications.