How to view the DB9 to USB port？？

if it's a standard usb to DB9 interface it will be wired
TX >>> TX
RX <<< RX

you will need to change the wiring at the DB9 end to
TX >>>>RX
RX <<<<TX

it's called a null modem cable or crossover cable.
they are not that common any more due to ethernet and wifi

Back in the day when we actually used serial ports, a RS232 breakout box was invaluable here, It let you monitor signals on both sides and connect them as required.

Hello, you friends who have paid attention to and helped me. Thank you for your answer.I'm very upset that I haven't solved this problem until now.I write down the steps of the methods I have tried to do below.Please check, thank you！

Hi ! Robertspark
Thank you very much for your comments.
I have followed your advice:
RX---TX，TX---RX
But Linux and windows still can't communicate. Instead, windows and windows communicate successfully! LOL

Hi !! Andy
I have read the links you provided in detail, and it seems that I can't find what I need (maybe my knowledge is limited)
Thank you!

I tried this method from the Internet, but it failed~~

First, short circuit RX and TX pins, let them test themselves, send and receive by themselves
Enter the command in the terminal：echo"HelloWorld" > /dev/ttyS0  &&  cat /dev/ttyS0

If the communication is successful, it will print back to "HelloWorld", but after I issue the command, the terminal has been waiting....

What is the result of:

ls -la /dev/ttyS*

This will give you the owner & group of the serial port. And if you have multiple ports you will have to try each port.

GtkTerm is a program that is similar in use to Windows Hypertem which may make life easier. Not knowing your particular distro of the Linux you'll have to work out the installation procedure. Just remember to turn off "local echo"

As I mentioned a fair few posts back you need to have the correct group membership to access the serial port. And I mentioned trying a loopback test quite a few posts back, with links to docs that explain this. I really wonder why I'm taking time out to help when you seem to ignore most of the stuff I'm posting.

Hi !! ALittleOffTheRails 

I'm very sorry for not replying to your message. This is my negligence. Please forgive me.

① When I type :    ls -la /dev/ttyS*
There are only four terminal displays, which are
/dev/ttyS0
/dev/ttyS1
/dev/ttyS2
/dev/ttyS3

② I tried the loopback test. I short the TX and Rx of the DB9 head of the Linux host.
Then enter in the terminal : "  echo "hello" > /dev/ttyS0 && cat /dev/ttyS0 "
Then the terminal has been waiting and there is no response. I tried ttys0-ttys3, but there was no response and no feedback
 

Hi !! ALittleOffTheRails
Because I've just come into contact with Linux for a short time, I'm puzzled by what you said:
“As I mentioned a fair few posts back you need to have the correct group membership to access the serial port.”

Do I need to regroup my user names? Do you mean this? Can I group my username and port together?