  A simulation of top quark mass measurement scheme near the $t\bar{t}$
production threshold in future $e^{+}e^{-}$ colliders, e.g. the Circular
Electron Positron Collider(CEPC), is performed. $\chi^2$ fitting method is
adopted to determine the number of energy points to be taken and their
locations. Our result shows that the optimal energy point is located near the
largest slope of the cross section to beam energy and the most efficient scheme
is to concentrate all luminosity on this single energy point in one parameter
top mass fitting case. This suggests that the so called data driven method can
be a best choice for the future real experimental measurement. Conveniently,
the top mass statistical uncertainty can also be calculated directly by the
error matrix even without any sampling and fitting. Agreement of the above two
optimization methods has been checked. Our conclusion is that by taking
50~$fb^{-1}$ total effective integrated luminosity data, the statistical
uncertainty of the top potential subtracted mass can be suppressed to about
7~MeV and the total uncertainty is about 30~MeV. This precision will help to
identify the stability of the electroweak vacuum at the Planck scale.
