  Quasi-periodic propagating intensity disturbances have been observed in large
coronal loops in EUV images over a decade, and are widely accepted to be slow
magnetosonic waves. However, spectroscopic observations from Hinode/EIS
revealed their association with persistent coronal upflows, making this
interpretation debatable. We perform a 2.5D magnetohydrodynamic simulation to
imitate the chromospheric evaporation and the following reflected patterns in a
flare loop. Our model encompasses the corona, transition region, and
chromosphere. We demonstrate that the quasi periodic propagating intensity
variations captured by the synthesized \textit{Solar Dynamics
Observatory}/Atmospheric Imaging Assembly (AIA) 131, 94~\AA~emission images
match the previous observations well. With particle tracers in the simulation,
we confirm that these quasi periodic propagating intensity variations consist
of reflected slow mode waves and mass flows with an average speed of 310 km/s
in an 80 Mm length loop with an average temperature of 9 MK. With the
synthesized Doppler shift velocity and intensity maps of the \textit{Solar and
Heliospheric Observatory}/Solar Ultraviolet Measurement of Emitted Radiation
(SUMER) Fe XIX line emission, we confirm that these reflected slow mode waves
are propagating waves.
