The coupling between the line defect and point defect in a two-dimensional photonic crystal is studied by using plane wave expansion method and finite difference time domain method. Based on the analysis, we propose a novel tunable terahertz narrow-band filter. Simulation results on the defect modes and output spectra show that the photonic crystal that consists of two line defects and one point defect can act as a tunable filter with narrow bandwidth of 0.02 THz under the control of an external electric field.