In order to achieve a large field of view for a 1.23m large telescope, the design and assembly of a near-IR telescope has been done in this paper. First, an on-axis reflected system was designed based on the optical theory. Secondly, by designing and analyzing the deformation of different support structures for the secondary mirror and the deformation of cylinder type and truss structure, the three-beam secondary mirror support structure and the truss structure were chosen. Last, the adjustment was done by a proper secondary mirror adjusting device and a proper CCD counter-rail. A test was done after the assembly and adjustment, with the result of RMS=0.10618 λ, PV=0.55362 λ（λ=632nm）. It is proved that the design of the near-IR system can satisfy the requirements of the optical design and imaging according to the test result.