The combination of photonic crystal technology and terahertz technology has provided a new idea to design the terahertz-wave modulator. A terahertz-wave modulator based on self-collimation effect and Fabry-Pérot (F-P) cavity is presented. It is made by filling 5CB liquid crystal into the air holes of the photonic crystal which is designed with special structures. The transmission characteristics of the THz wave, including time-domain response, frequency-domain response and energy distributions of the steady state, are studied with the finite-difference time-domain method (FDTD). The modulator, with a modulation rate of about 10 kHz, an insertion loss of about 0.308 dB, and a tuning range of 0.308～34.32 dB, has a small size. It can be designed according to the demand of the working wavelength and can be easily integrated. Wide operating wavelength range is the greatest advantage of this modulator. It is an ideal terahertz-wave modulator.