Hi, I tried another way now my kindle worked well. I'm not naive English speaker, so please forgive my bad English speaking skill.
First, get uImage file from update_kindle_8th_5.8.5.bin, and use command loady to send it to kindle. My uImage file size is 4778496, blk len is 512, so 4778496/512=9333=0x2475
Second, execute command mmc write 0x80800000 0x208 0x2475
Now, press your kindle power button until kindle reboot, then you can entry recovery system, select e to export fat partition, and copy update_kindle_8th_5.8.5.bin into it, enter 1, and select u, wait for some time, your kindle 8th will back ^_^
thanks for your help