hello daojin.xu101,
please refer to developer guide, Updating the Rootfs Partition with a Customized Updater and Image, or.. you may replace “system.img.raw” with your rootfs image.
BTW,
did you working with a customize board?
if yes, please see-also Topic 332980 to update l4t_generate_ota_package.sh and also ota_board_specs.conf for supporting your customize board.