A novel terahertz modulator based on photonic crystal is presented.The modulator combines the structure of line defect and point defect,and is designed to be tunable by filling the point defect with nonlinear material GaAs.Without external pumplight,there is defect mode in the point defect.While by applying external pumplight to the defect with specific wavelength and intensity,the defect mode in the two defects \soutwill disappears.Based on this mechanism,the device can modulates the onoff state of the terahertz wave in specific wavelength.The numerical simulation results show that this THz modulator has a extinction ratio of more than 5110 dB,responses within dozens of picoseconds,and has a small size and is easy to integrate.