@jock @Asriel Nguyen finally I found the solution.
Rockchip has an official document say:
https://opensource.rock-chips.com/wiki_Rockusb#Maskrom_mode
So, the command of write u-boot-main.img is
rkdeveloptool wl 0x2000 u-boot-main.img
After write u-boot, you able to boot MultiTool from OTG USB