Launching ultrashort femtosecond photoacoustic pulses with multi-terahertz bandwidth will find broad applications from fundamental acoustics in 2D materials and THz-acoustic and phonon spectroscopy to nondestructive detection in opaque materials with a sub-nanometer resolution. Here we report the generation of ultra-short 344 fs photoacoustic pulses with a 2.1 THz bandwidth from interfacial two-dimensional electron gas using optical femtosecond excitation. A comparison with simulation supports the dominant contribution of hot electron pressure and the ultrafast electron relaxation to produce pulsewidth shorter than the acoustic transit time across the electron wavefunction. Our simulation further indicates the possibility to generate <200 fs photoacoustic pulse.