A terahertz focal-plane phase imaging method with multiwavelength is presented. This novel approach can image object with larger optical length compared to using the largest wavelength in the terahertz spectrum and does not involve the usual phase unwrapping in the detection of phase discontinuity. Furthermore, this technique can effectively reduce the noise background and improve the accuracy of the reconstructive images. The validity of this new method can be demonstrated by processing the reflective two dimensional (2D) image data of a metal washer and Nikon camera’s lens cap. The measurements show that the multiwavelength phase imaging is a straightforward and efficient phase data processing method in terahertz imaging application.