Terahertz (THz) wave modulator is a key element in future terahertz communication systems. Meanwhile, as a new kind of material, the tunable photonic crystal is widely used to fabricate modulators, switches, filters, and other functional devices in optical communication systems. The combination of THz technology and photonic crystal technology provides a new approach to design the novel THz wave modulator. A novel THz modulator based on the twodimensional silicon photonic crystals is presented. The structure of photonic crystals is consisted of the combination of line defects and point defects, and is designed to be the tunable photonic crystals by filling the point defect with 5CB liquid crystal. By applying the external electrical field on the point defect, the frequency of the defect mode of the THz photonic crystals dynamically shifts. Based on this mechanism, the device can modulate the onoff state of the THz wave propagating. The numerical simulating results show that the THz modulator has a modulation depth of more than 30dB, modulation rate of 10 kHz, and a small size, ease to integrate.