Jiao Tong University Spectroscopic Telescope(JUST)is a 4.8-m composite spectroscopic telescope which inte grates the high-resolution observation in a small field of view(FoV)with the spectral survey in a wide FoV. The primary mirror of JUST adopts the latest segmented mirror active optics technology,which is composed of 18 hexagonal segments with a diameter of 1.1 meters each.There are two Nasmyth focal stations which can be switched by rotating the tertiary mirror.One station with a 10 arcmin FoV diameter and its image quality (full width at half maximum,FWHM)can be below 0.35 arcsec with the error budgets,which is suitable for in stalling high-resolution spectroscopic and imaging instruments.The other station is equipped with the wide-field correctors,which can achieve a FoV diameter of scientific 1.2 degrees and correct atmospheric dispersion.The wide-field Nasmyth focal station operates at a wavelength band ranging from 0.35 to 1.3µm and for the zenith distance ranging from 0 to 60 degrees,the image quality(FWHM)falls within a 0.75 arcsec diameter with error budgets.It is used for broad observations and mainly used for wide-field spectroscopic survey of stars,galaxies, etc.This article will describe the optical system design,the baffle design and stray light analysis,error budget, and thermal effect of the telescope in detail.