Thank you very much, Aaronwu!
In arch/blackfin/lib/board.c, there is:
dcplb_add(CONFIG_SYS_FLASH_BASE, PAGE_SIZE_16MB | CPLB_DIRTY | CPLB_SUPV_WR |
CPLB_USER_WR | CPLB_USER_RD | CPLB_VALID);
I changed "PAGE_SIZE_16MB" to PAGE_SIZE_64MB", rebuild uboot, and it now works fun.