  The escaping atmospheres of hydrogen driven by stellar X-ray and extreme
Ultraviolet (XUV) have been detected around some exoplanets by the excess
absorption of Ly$\alpha$ in far ultraviolet band. In the optical band the
excess absorption of H$\alpha$ is also found by the ground-based instruments.
However, it is not certain so far if the escape of the atmosphere driven by XUV
can result in such absorption. Here we present the XUV driven hydrodynamic
simulation coupled with the calculation of detailed level population and the
process of radiative transfer for WASP-121b. Our fiducial model predicts a mass
loss rate of $\sim$1.28$\times$10$^{12}$g/s for WASP-121b. Due to the high
temperature and Ly$\alpha$ intensity predicted by the fiducial model, many
hydrogen atoms are populated into the first excited state. As a consequence,
the transmission spectrum of H$\alpha$ simulated by our model is broadly
consistent with the observation. Comparing with the absorption of H$\alpha$ at
different observation times, the stellar XUV emission varies in the range of
0.5-1.5 times fiducial value, which may reflect the variation of the stellar
activity. Finally, we find that the supersonic regions of the planetary wind
contribute a prominent portion to the absorption of H$\alpha$ by comparing the
equivalent width of H$\alpha$, which hints that a transonic outflow of the
upper atmosphere driven by XUV irradiation of the host star can be detected by
the ground-based telescope and the H$\alpha$ can be a good indicator of
escaping atmosphere.
