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My external hard disk is 1TB in size and nearly 900GB data on it. I want to set up a password without format it. Is it possible?
file format is ext4.

Yes that's possible.

That's called ATA password. You can either set it through your EFI/BIOS firmware, if it allows you to do so. Or you can use hdparm to set such a password.

I would recommend to use your EFI firmware if applicable because hdparm comes with a lot of warning that it can brik your disk. That's kind of scary, even it is unlikely to happen. That are just cautious and warn you of the worst.

So if you want to use hdparm, it's something like:

sudo hdparm --user-master u --security-set-passwd "Secret" /dev/sdx

That is to set a user password 'Secret'. This will let the master password as set by the manufacturer (and let anyone knowing it unlock the drive).

You can eventually also set a master password. But read documentation before to know the consequences.

A good read is this admin-magazine.com article

Note: if the drive with the ATA password is plugged-in while booting, your EFI firmware should ask you for that password. If you plugged it in after booting, I don't know how it is dealt with by ubuntu, but you might have to unlock the drive by issuing another hdparm command from a terminal, like

sudo hdparm --security-unlock 'Secret' --user-master u /dev/sdx

if we use the same variable in our previous example.

Another possibility is to use ext4 built-in encryption, but it only encrypts per directory. Meaning, if you have multiple top directory on your HDD, you will need a password for each directory, unless you move them under the same top root directory. More instruction in the archlinux wiki on how to enable ext4 encryption. This works on new files, so you will have to move all your data to the new encrpyted directory, just to encrypt them.