In this study， the gene encoding cellobiose 2-epimerase from Caldicellulosiruptor saccharolyticus (CsCE) was successfully expressed in Bacillus subtilis WB800. After the fermentation medium optimization， the activity of recombinant strain was 4.5-fold higher than the original medium in a 7.5L fermentor. The optimal catalytic pH and temperature of crude CsCE were 7.0 and 80°C， respectively. An enzymatic synthesis of lactulose was developed using cheese-whey lactose as its substrate. The maximum conversion rate of whey powder obtained was 58.5% using 7.5 U/mL CsCE. The enzymatic membrane reactor system exhibited a great operational stability， confirmed with the higher lactose conversion (42.4%) after 10 batches. To our best knowledge， this is the first report of lactulose synthesis in food grade strain， which improve the food safety， and we not only realize the biological production of lactulose， but also make good use of industrial waste， which have positive impact on environment.