With the development of continuous-wave terahertz (THz) sources and array detectors, the pursuit of high-fidelity real-time imaging is receiving significant attention within the THz community. Here, we report a real-time full-field THz phase imaging approach based on lensless Fourier-transform THz digital holography. A triangular interferometric layout is proposed based on an oblique illumination of 2.52 THz radiation, which is different from other lensless holographic configurations at other frequencies. A spherical reference beam is generated by a reflective parabolic mirror with minor propagation loss. The complex-valued images are reconstructed using a single inverse Fourier transform of the hologram without complex calculation of the diffraction propagation. The experimental result for a Siemens star validates the lateral resolution of ∼346μm in the diagonal direction. Sub-pixel image registration and image stitching algorithms are applied to enlarge the area of the reconstructed images. The dehydration process of an aquatic plant leaf (Hottonia inflata) is monitored for the first time, to the best of our knowledge, at the THz band. Rapid variations in water content and morphology are measured with a time interval of 0.6 s and a total time of 5 min from a series of reconstructed amplitude and phase images, respectively. The proposed method has the potential to become a powerful tool to investigate spontaneous phenomena at the THz band.