BEIJING—China's biggest state-owned banks reported a surge in soured and castoff loans in the first half, as China's slowing growth takes a toll on its lenders.

The five biggest state-owned banks—Industrial & Commercial Bank of China Ltd., China Construction Bank Corp., Agricultural Bank of China Ltd., Bank of China Ltd. and Bank of Communications Co.—wrote off and transferred out of their books a total of 46.91 billion yuan ($7.64 billion) of bad loans in the first half, according to calculations by The Wall Street Journal....