  Based on $J/\psi$ events collected with the BESIII detector, with
corresponding Monte Carlo samples, the tracking efficiency and its systematic
uncertainty are studied using a control sample of $J/\psi \to p \overline p
\pi^+ \pi^-$. Validation methods and different factors influencing the tracking
efficiency are presented in detail. The tracking efficiency and its systematic
uncertainty for protons and pions with the transverse momentum and polar angle
dependence are also discussed.
