The shape is the most important characteristic of the big blue bump (bbb). By employing the sample from Walter & Fink, and supplementing it with medium energy X-ray data, we have been successful in separating the bbb component from the nonthermal component for the soft X-ray region and thus obtaining the cutoff energy E cut of the big bump in the XUV region. Our sample contains 30 type I Seyfert galaxies, which is the largest sample size for the determination of the bbb shape. Statistical

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... es show a significant anticorrelation between the spectral UV index (b uv ) and E cut . The thermal emission mechanisms for the bbb include the optically thick thermal radiation from the accretion disk (and its corona region), the optically thin thermal free-free radiation and other models. The statistical properties between b uv and E cut , provide an important base for picking and improving a mechanism.