$R$ measurement and QCD studies at future super $\tau-c$ factory

We review status of $R$ measurement and QCD studies at low energy range, discuss prospects for a super $\tau-$charm factory in $2-7$ GeV. With a high-luminosity $e^+e^-$ collider, statistics are no longer problem for $R$ measurement and a precision of $2\%$ or even better is foreseen, that will lead to bring down the uncertainty of hadronic contribution to the QED running coupling constant $\Delta\alpha_{had}$ and the anomalous magnetic moment of the muon ($a_{\mu}$); measure the strong coupling constant $\alpha_s$ and the charm quark mass; improve the measurement of the resonance parameters of heavy charmonia. Huge data samples in $2-3$ GeV will make it possible to study excited states of $\rho$, $\omega$ and $\phi$, or exotic $Y(2175)$; measure electromagnetic form factor of mesons and baryons; and measure fragmentation functions of hadrons.