how to download x-loader,u-boot,kernel on the board which only left uart and usb?

If uboot is already flashed then u can use usb interface to download images from connected MSC stick.

Regards,
Ajay

hello!

   thank you very much! can you tell me how to operate the specific, or give me a web site?

You can refer below page,

http://processors.wiki.ti.com/index.php/AM35x-OMAP35x-PSP_03.00.00.05_UserGuide#MUSB_Host_Support

Regards,
Ajay

I have  looked for your wed site ,yes , in uboot environmen can use USB download ,but how to use  UART and USB download x-loader and uboot ,can achieve?

If uboot is corrupted then USB can't be used to download/update x-load/uboot binaries. I think you can use UART kermit command to transfer binaries in this case on OMAp35x.

Ajay

Also you can use JTAG interface (if available) to download and execute x-loader/u-boot to get u-boot prompt and then flash it.

Thanks,

Vaibhav