Deng, H.X. JACoW Publishing Geneva, Switzerland We report a novel self-amplification method for enhancing laser-induced energy modulation, thereby significantly reducing requirements on the external laser system in a seeded free-electron laser (FEL). Driven by this scheme, we experimentally realized high harmonic generation in a seeded FEL using an unprecedentedly small external laser-induced energy modulation. An electron beam with the laser-induced energy modulation as small as 1.8 times the slice energy spread has been used for lasing at the seventh harmonic of 266-nm seed laser in a single-stage high-gain harmonic generation (HGHG) setup and the 30th harmonic of the seed laser in a two-stage HGHG setup. The results mark a major step towards a high-repetition-rate, fully coherent X-ray FEL.