Hello 13226508@qq.com,
Well, is strange if the data transmission works using your external USB to Serial after the elf started to run, but on the bootloader is not ok. Normally, the serial connection is point to point so in any case, adding another usb to serial on the same line should cause problems on the line.
To help you, I can propose another way, already discussed here: Why does the example model from MBDT not work?. If you have no constraints regarding the pins, you can use the LIN0(PB2 and PB3) instead LIN1. For that you simply flash on the board the MPC5744P.rbf file located in src\tools\BootLoader\RBF_Files, and use the PB2 and PB3 pins.
For more info please follow the thread mentioned.
Hope this helps,
Marius