  Magnetic reconnection is the key mechanism for energy release in solar
eruptions, where the high-temperature emission is the primary diagnostic for
investigating the plasma properties during the reconnection process.
Non-thermal broadening of high-temperature lines has been observed in both the
reconnection current sheet (CS) and flare loop-top regions by UV spectrometers,
but its origin remains unclear. In this work, we use a recently developed
three-dimensional magnetohydrodynamic (MHD) simulation to model magnetic
reconnection in the standard solar flare geometry and reveal highly dynamic
plasma flows in the reconnection regions. We calculate the synthetic profiles
of the Fe XXI 1354 \AA~line observed by the Interface Region Imaging
Spectrograph (IRIS) spacecraft by using parameters of the MHD model, including
plasma density, temperature, and velocity. Our model shows that the turbulent
bulk plasma flows in the CS and flare loop-top regions are responsible for the
non-thermal broadening of the Fe XXI emission line. The modeled non-thermal
velocity ranges from tens of km s$^{-1}$ to more than two hundred km s$^{-1}$,
which is consistent with the IRIS observations. Simulated 2D spectral line maps
around the reconnection region also reveal highly dynamic downwflow structures
where the high non-thermal velocity is large, which is consistent with the
observations as well.
