  By varying the profiles of stellar extreme ultraviolet (EUV) spectral energy
distribution (SED), we tested the influences of stellar EUV SEDs on the
physical and chemical properties of the escaping atmosphere. We apply our model
to study four exoplanets, HD\,189733b, HD\,209458b, GJ \,436b, and Kepler-11b.
We found that the total mass loss rates of an exoplanet, which are determined
mainly by the integrated fluxes, are moderately affected by the profiles of the
EUV SED, but the composition and species distributions in the atmosphere can be
dramatically modified by the different profiles of the EUV SED. For exoplanets
with a high hydrodynamic escape parameter ($\lambda$), the amount of atomic
hydrogen produced by photoionization at different altitudes can vary by one to
two orders of magnitude with the variation of stellar EUV SEDs. The effect of
photoionization of H is prominent when the EUV SED is dominated by the
low-energy spectral region (400-900${\AA}$), which pushes the transition of
H/H$^{+}$ to low altitudes. On the contrary, the transitions of H/H$^{+}$ move
to higher altitudes when most photons concentrate in the high-energy spectral
region (50-400${\AA}$). For exoplanets with a low $\lambda$, the lower
temperatures of the atmosphere make many chemical reactions so important that
photoionization alone can no longer determine the composition of the escaping
atmosphere. For HD 189733b, it is possible to explain the time variability of
\lya\ between 2010 and 2011 by a change in the EUV SED of the host K star, yet
invoking only thermal H\,I in the atmosphere.
