  Weak-line quasars (WLQs) are a notable group of active galactic nuclei (AGNs)
that show unusually weak UV lines even though their optical-UV continuum shapes
are similar to those of typical quasars. The physical mechanism for WLQs is an
unsolved puzzle in the AGN unified model. We explore the properties of UV
emission lines by performing extensive photoionization calculations based on
Cloudy simulation with different spectral energy distributions (SEDs) of AGNs.
The AGN continua are built from several observational empirical correlations,
where the black-body emission from the cold disk, the power-law emission from
the hot corona, and a soft X-ray excess component are considered. We find that
the equivalent width (EW) of C {\footnotesize IV} from our models is
systematically lower than observational values if the component of soft X-ray
excess is neglected. The EW will increase several times and is roughly
consistent with the observations after considering the soft X-ray excess
component as constrained from normal type I AGNs. We find that the UV lines are
weak for QSOs with quite large BH mass (e.g., $M_{\rm BH}>10^9M_{\odot}$) and
weak soft X-ray emission due to the deficit of ionizing photons. As an example,
we present the strength of C {\footnotesize IV} based on the multi-band SEDs
for three nearby weak-line AGNs, where the weaker soft X-ray emission normally
predicts the weaker lines.
