The generation and detection of narrowband terahertz (THz) wave based on photorefractive periodically poled lithium niobate (PPLN) and periodically poled Mg:LiNb₃(PP-Mg:LN) crystal are realized by optical recti fication.By changing the size and direction of the applied magnetic field,the effective mod ulation on the amplitude of the narrowband THz wave is achienved in photorefractive PPLN crysta l.With the increase of the applied magnetic field,the amplitude of narrowband THz wave in the photorefractive PPLN crystal is supressed. When the applied magnetic field is sufficiently strong,narrowband THz wave in t he photorefractive PPLN crystal can be completely suppressed.For PP-Mg:LN cryst al,a completely different phenomenon of the THz wave is observ ed.The oscillation of the generated THz wave does not produce any changes,either with or without the magnetic field.The magnetic field modulation of THz wave in the photorefractive PPLN crystal comes from the buildup of a space-charge fiel d through Lorentz force.