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 $alpha_{rm NUV}$ and $alpha_{rm FUV}$ 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 $alpha_{rm NUV}$ and $alpha_{rm FUV}$ have a common upper limit ($sim 1/3$) which is almost independent of the quasar luminosity $L_{rm bol}$. 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 {it locally}. With this assumption, we employ the dust extinction scenario to model the observed $L_{rm bol}-alpha$ relation. We find that, a typical value of $E(B-V)sim0.1$ to $0.3$ mag (depending on the types of extinction curve) of the quasar {it local} dust is enough to explain the discrepancy of $alpha$ between the observation ($sim-0.5$) and the standard accretion disk model prediction ($sim 1/3$).
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