The transverse mode distribution of THz wave near the position of focus point was studied in this paper. It is important to know the transverse distribution of THz wave, which can be referenced as an evidence of sample positioning in THz spectroscopy and THz imaging experiments. By using THz point-by-point imaging technology, THz transverse mode distribution in the positions, where samples are usually placed, is analyzed qualitatively. By applying “slit method”, the beam width of THz wave was measured. The results show that the shapes of the transverse modes of THz wave in different positions around the focus have much difference. To the maximum wavelength of about 1.5 mm of THz wave in the system, the minimum diameter of THz beam at the focus point is around 1.0 mm. The results provide a basic reference for further application of THz Spectroscopy and THz Imaging.