  We established a method on measuring the $\dzdzb$ mixing parameter $y$ for
BESIII experiment at the BEPCII $e^+e^-$ collider. In this method, the doubly
tagged $\psi(3770) \to D^0 \overline{D^0}$ events, with one $D$ decays to
CP-eigenstates and the other $D$ decays semileptonically, are used to
reconstruct the signals. Since this analysis requires good $e/\pi$ separation,
a likelihood approach, which combines the $dE/dx$, time of flight and the
electromagnetic shower detectors information, is used for particle
identification. We estimate the sensitivity of the measurement of $y$ to be
0.007 based on a $20fb^{-1}$ fully simulated MC sample.
