  The Circular Electron Positron Collider (CEPC) with a center-of-mass energy
$\sqrt{s}$ = 240 GeV is proposed to serve as a Higgs factory, while it can also
provide good opportunity for new physics searches at lower energy, which are
difficult in hadron colliders but well-motivated by some theory models such as
dark matter. This paper presents the sensitivity study of direct stau / smuon
production at CEPC using full Monte Carlo (MC) simulation. With the assumption
of flat 5% systematic uncertainty, the CEPC has the potential to discover the
production of combined left-handed and right-handed stau up to 116 GeV if
exists, or up to 113 GeV for the production of pure left-handed / right-handed
stau; the discovery potential of direct smuon reaches up to 117 GeV with the
same assumption. Due to the conserved assumption of systematic uncertainty and
limited reliance on the reconstruction and detector geometry in this study, the
results can be used as reference for similar searches in other electron
positron colliders at a center-of-mass energy close to 240 GeV, such as Future
Circular Collider $e^{+}e^{-}$ (FCC-ee) and the International Linear Collider
(ILC).
