According to the requirements of THz imaging detection, an optical system suitable for the THz waveband is designed. The optical system has a focal length of 50 mm, a F number of 1, an operation waveband from 50 μm to 100 μm and a full field of view of 11.9°. Its receiver is an uncooled IRM160A infrared detector. In the system, a coaxial structure consisted of three spherical lenses and a piece of glass for protecting a CCD is used. A new material Topas COC is used. In addition, to meet the requirements of simplicity, low weight and low cost, an aspheric surface is also introduced. The design result shows that at a cut-off frequency of 10 cy/mm, the optical system has a modulation transfer function close to the diffraction limited curve. In each field of view, the RMS radii of the spot diagrams are far less than the airy disk radius. Its imaging quality is good. So it can meet the overall design requirements of a THz optical system.