Kinematics of tau two-body decay near tau threshold at BESIII

The kinematic properties of two-body decay near tau threshold are studied according to the special capacity of the BEPC II accelerator and the BESIII detector. Explicitly presented are the transformations of energy and momentum of hadronic particles between different reference frames, and the corresponding distributions. A brand new method is proposed to obtain the energy spread of the accelerator by fitting the energy distribution of hadron from tau semi-leptonic decays.