  The $pp\to p n \pi^+$ reaction is a channel with the largest total cross
section for pp collision in COSY/CSR energy region.
  In this work, we investigate individual contributions from various $N^*$ and
$\Delta^{*}$ resonances with mass up to about 2 GeV for the $pp\to p n \pi^+$
reaction. We extend a resonance model, which can reproduce the observed total
cross section quite well, to give theoretical predictions of various
differential cross sections for the present reaction at $T_p=2.88$ GeV. It
could serve as a reference for identifying new physics in the future
experiments at HIRFL-CSR.
