  The Dalitz plot of $\eta^{\prime} \to \eta \pi^+ \pi^-$ decay is studied
using $(225.2\pm2.8)\times 10^6$ $\jpsi$ events collected with the BESIII
detector at the BEPCII $e^+e^-$ collider. With the largest sample of
$\eta^{\prime}$ decays to date, the parameters of the Dalitz plot are
determined in a generalized and a linear representation. Also the branching
fraction of $\jpsi \to \gamma \eta^{\prime}$ is determined to be
$(4.84\pm0.03\pm0.24)\times 10^{-3}$, where the first error is statistical and
the second systematic.
