Oh... It now works, it seems like I used a proxy for the ppa server cause I can't reach launchpad site in China. Now it works but the upgrader said it will remove some third-party ppa repositories but it's okay I can add them back after the upgrade.
Thanks very much.
Edit: After changing back the ppa URLs to ppa.launchpad.net, the simulated .list file changed from "focal" to "jammy" correctly. It seems that the upgrader used a replace command to change focal to jammy but it considered other content before the word "focal" so it didn't replace my proxied url proxy.ustclug.org (I used this replaced the default ppa url because I can't reach launchpad in China). Upgrader can use some other replace commands only change the word "focal" to "jammy" or just put the package build "zorin-os-16-to-17-upgrade" into the ppa repository of the focal version. Thanks.