We investigate the UV continuum slope \alpha of a large quasar sample from SDSS DR7 .
By using specific continuum windows , we build two samples at lower ( 0.71 < z < 1.19 ) and higher ( 1.90 < z < 3.15 ) redshifts , which correspond to the continuum slopes at longer ( NUV ) and shorter ( FUV ) rest wavelength ranges respectively .
Overall , the average continuum slopes are -0.36 and -0.51 for \alpha _ { NUV } and \alpha _ { FUV } with similar dispersions \sigma _ { \alpha } \sim 0.5 .
For both samples , we confirm the luminosity dependence of the continuum slope , i.e. , fainter quasars have redder spectra .
We further find that both \alpha _ { NUV } and \alpha _ { FUV } have a common upper limit ( \sim 1 / 3 ) which is almost independent of the quasar luminosity L _ { bol } .
This finding implies that the intrinsic quasar continuum ( or the bluest quasar ) , at any luminosity , obey the standard thin disk model .
We propose that the other quasars with redder \alpha are caused by the reddening from the dust locally .
With this assumption , we employ the dust extinction scenario to model the observed L _ { bol } - \alpha relation .
We find that , a typical value of E ( B - V ) \sim 0.1 to 0.3 mag ( depending on the types of extinction curve ) of the quasar local dust is enough to explain the discrepancy of \alpha between the observation ( \sim - 0.5 ) and the standard accretion disk model prediction ( \sim 1 / 3 ) .