when I setup owncloud on the server of ubuntu16.04,I USE root to set,
this result that I can't change the document without the account of root,
so I want to know how to change the owner of owncloud
I am not sur I understand your question. What do you mean by "the owner of owncloud" ?
Do you mean the owner of the install? On Ubuntu, if you are using Apache, the owner of the install should be the apache agent www-data.
Do you mean the user on the database ? You can create another user and modify accordingly the config
Or do you mean the owner of the data files ? you could have a look at the forum
https://forum.owncloud.org/viewtopic.php?f=23&t=11270
There is no direct possibility. the simplest way seems to copy the files locally in another directory, and create a new sync with a new user. And then delete everything that belongs to the user root.
If you like taking some risk, you could also go in the database (for instance using phpmyadmin if you use mysql) and manually modify the ownership of the files. But be sure to have an available backup to come back in a working situation