If in the telecentric off-axis three-mirror imaging system there is a beam-limited device in front of space remote instrument, a real entrance pupil to match is required. To meet this need, optical design of off-axis three-mirror system whose aperture stop coincides with the object focal plane in front of the primary mirror is studied. The expressions of configuration parameters and the interval between the object focus and the primary mirror of off-axis three-mirror system are deduced in case that the primary mirror and the tertiary mirror are coplanar. The condition and solution of achieving real object focus are defined. Based on the method, with the specific requirements of 1.0～2.5 μm spectral range, F-number 4, 360 mm focal length, 5° field of view (FOV), a telecentric off-axis three-mirror system with real entrance pupil is designed. And the imaging quality of system approaches the diffraction limit at full FOV in the required wave band. The highest aspheric order is four. The result indicates that this method can be used in design of telecentric off-axis three-mirror imaging system with real entrance pupil.