  By combining the newly infrared photometric data from the All-Sky Data
Release of the Wide-field Infrared Survey Explorer with the spectroscopic data
from the Seventh Data Release of the Sloan Digital Sky Survey, we study the
covering factor of warm dust ($\CF$) for a large quasar sample, as well as the
relations between $\CF$ and other physical parameters of quasars. We find a
strong correlation between the flux ratio of mid-infrared to near-ultraviolet
and the slope of near-ultraviolet spectra, which is interpreted as the dust
extinction effect. After correcting for the dust extinction utilizing the above
correlation, we examine the relations between $\CF$ and AGN properties:
bolometric luminosity ($\Lbol$), black hole mass ($\MBH$) and Eddington ratio
($L/L_{\rm Edd}$). We confirm the anti-correlation between $\CF$ and $\Lbol$.
Further we find that $\CF$ is anti-correlated with $\MBH$, but is independent
of $L/L_{\rm Edd}$. Radio-loud quasars are found to follow the same
correlations as for radio-quiet quasars. Monte Carlo simulations show that the
anisotropy of UV-optical continuum of the accretion disc can significantly
affect, but is less likely to dominate the $\CF$--$\Lbol$ correlation.
