We demonstrate picosecond terahertz (THz)-wave generation via four-wave mixing in an octagonal photonic crystal fiber (O-PCF). Perfect phase-matching is obtained at the pump wavelength of $1.55\mathrm{\mu m}$ and a generation scheme is proposed. Using this method, THz waves can be generated in the frequency range of $7.07–7.74\mathrm{THz}$ . Moreover, peak power of $2.55\mathrm{W}$ , average power of $1.53\mathrm{mW}$ , and peak conversion efficiency of more than $-66.65\mathrm{dB}$ at $7.42\mathrm{THz}$ in a $6.25\mathrm{cm}$ long fiber are realized with a pump peak power of $2\mathrm{kW}$ .