  A warm corona has been widely proposed to explain the soft X-ray excess (SE)
above the 2--10 keV power law extrapolation in AGNs. In actual spectral
fittings, the warm coronal seed photon temperature ($T_{\rm s}$) is usually
assumed to be far away from the soft X-ray, but $kT_{\rm s}$ can reach close to
0.1 keV in standard accretion disc model. In this study, we used Monte Carlo
simulations to obtain radiation spectra from a slab-like warm corona and fitted
the spectra using the spherical-geometry-based routine \textsc{thcomp} or a
thermal component. Our findings reveal that high $T_{\rm s}$ can influence the
fitting results. A moderately high $kT_{\rm s}$ (around 0.03 keV) can result in
an apparent low-temperature and flat SE, while an extremely high $kT_{\rm s}$
(around 0.07 keV) can even produce an unobserved blackbody-like SE. Our
conclusions indicate that, for spectral fittings of the warm coronal radiation
(SE in AGNs), $kT_{\rm s}$ should be treated as a free parameter with an upper
limit, and an accurate coronal geometry is necessary when $kT_{\rm s}>0.01$
keV.
