Abstract: Terahertz(THz) waves occupy a very important position in the electromagnetic spectrum. THz technology is recognized as a major cross-cutting frontier in the international scientific and technology community. Photonic generation of THz signal using a recirculating frequency-shifting loop(RFSL) was proposed and experimentally demonstrated. The frequency of optical source was shifted by the RFSL and beats with each other in a high-speed photodetector to generate a THz signal. In the RFSL, the single sideband(SSB) modulator driven by a radio frequency(RF) signal was a key component for frequency-shifting. The frequency multiplication factor(FMF) of the THz signal was controlled by the lap number circulating in the loop. The frequency of the THz signal increased with the improvement of FMF and equals to the frequency multiplication factor multiplied by the frequency of RF signal. Experimentally, tunable signals from 5 GHz to 20 GHz were generated and the FMF was successfully tuned from 1 to 25. The stability of the power and center frequency was detected when the frequency of the signal was 20 GHz. 0.25 THz signal was finally generated by this system.