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 Ecut 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 analyses show a significant anticorrelation between the spectral UV index (buv) and Ecut. 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 buv and Ecut, provide an important base for picking and improving a mechanism.