Abstract: The light path of digital holographic microscope in line is designed to meet the requirements of high spatial resolution and big field of view which are the key parameters of the movable quantum cascade laser-terahertz (QCL-THz) digital holography imager. The numerical aperture of the microscope objective is 0.707 or 0.8, the amplification times is 6 and the optimal working distance is 15 mm. It can be achieved that the field of view is 10 mm and the spatial resolution is 100 micron in this light path using terahertz source whose wavelength is 96.8 microns.