Broadband tunability is a key pursuit for terahertz (THz) free-electron lasers, yet it is often constrained by the narrow energy acceptance of laser modulators in prebunched schemes. Here, we propose a harmonic modulation scheme to decouple this constraint. Taking the Electron Source Test Facility at Dalian as a specific case study, our simulations demonstrate that by matching the laser field to higher-order harmonic resonances of the undulator, we can obtain efficient energy modulation across a significantly wider electron energy range (40–130 MeV), surpassing the limit of 71–130 MeV imposed by the fundamental resonance. This technique facilitates continuous frequency tuning from 0.5 to 30 THz using a single modulator-radiator chain, establishing a versatile method for ultra-broadband THz generation.