Polyimide-modified carbon nanotubes (PI/CNTs) were synthesized via a solvent-free thermal method and used as a metal-free catalyst to activate peroxymonosulfate for organic contaminant degradation without light irradiation. The characterization results suggested that PI was loaded onto the surface of CNTs. The catalytic ability of the PI/CNTs was strongly correlated with the content of PI in the catalysts. The PI/CNTs (22% of PI) showed the highest catalytic efficiency for organic pollutant degradation at room temperature. The degradation efficiency of acid orange 7 (AO7) dye was significantly enhanced to 98.9% within 15 min, compared to the efficiency of 2.2% exhibited by pure PI. The radical quenching tests and electron paramagnetic resonance spectrometry proved that singlet oxygen, instead of hydroxyl radicals or sulfate radicals, played a dominant role during the catalytic oxidation of AO7. The influences of operation parameters including temperature and catalyst amount were investigated. The PI/CNTs metal-free catalyst exhibited high catalytic activity under a broad range of pH values. The recycling study of four repeated reactions demonstrated good stability of the PI/CNTs. This work provided a promising metal-free catalyst for degradation of organic pollutants in aqueous solutions, contributing to the development of green materials for sustainable remediation.