  The solar Ly$\alpha$ line is the strongest line in the ultraviolet waveband,
and is greatly enhanced during solar flares. Here we present radiative
hydrodynamic simulations of solar flares under different heating models, and
calculate the response of this line taking into account non-equilibrium
ionization of hydrogen and partial frequency redistribution. We find that in
non-thermal heating models, the Ly$\alpha$ line can show a red or blue
asymmetry corresponding to the chromospheric evaporation or condensation,
respectively. The asymmetry may change from red to blue if the electron beam
flux is large enough to produce a significant chromospheric condensation
region. In the Ly$\alpha$ intensity lightcurve, there appears a dip when the
change of asymmetry occurs. In thermal models, the Ly$\alpha$ line intensity
peaks quickly and then falls, and the profile has an overall red asymmetry,
which is similar to the profiles from heating by a soft electron beam. The
Ly$\alpha$ profile shows a single red peak at the end of thermal heating, and
the whole line is formed in a very small height range.
