Astrophysics and cosmology in the coming decades urgently need a large field-of-view (FOV), highly multiplexed spectroscopic survey telescope satisfying challenging image quality and stability requirements. The 6.5 m MUltiplexed Survey Telescope (MUST) proposed by Tsinghua University will be constructed on the Saishiteng Mountain of Northwest China to improve the spectroscopic survey capability of ground-based optical telescopes. In this paper, we demonstrate the conceptual design of the optical system of MUST. MUST will adopt a 6.5 m primary mirror, a 2.45 m secondary mirror, and a multiple-element widefield corrector (WFC) to ensure excellent image quality with an 80% encircled energy size of image spots less than ~ 0.6 arcsec in diameter for the entire 3° FOV and the whole 50° zenith angle range. Thanks to its compact 6.5 m Ritchey-Chretien system and 20,000 optical fibers on its Cassegrain focus, MUST will carry out state-of-the-art wide-field spectroscopic surveys with efficiency ~ 19 times higher than the Dark Energy Spectroscopic Instrument (DESI) using a measure proposed by Ellis et al. Upon completion around 2029, MUST will be one of the world's most advanced wide-field spectroscopic survey telescopes and a new essential reference for the future development of wide-field survey telescopes. It will enable significant advances in many fields in astrophysics and cosmology.