  In order to obtain the mass of $\tau$ lepton at BESIII precisely, a beam
energy measurement system was built at BEPCII. A scenario for high precision
$\tau$ mass measurement was put forth before data taken. More than 130
$pb^{-1}$ $\tau$ mass scan data were collected in April 2018, and the
uncertainty of $m_{\tau}$ is expected to be less than 100 keV.
