BuddyPress.org

@xiayou, from looking at your website, it looks like you are using a custom theme. The first thing that comes to mind, is Are you using the BP template pack?

Are you using a host that is free, or do you have a subscription?

If you have a subscription, it is most likely not the host, more likely conflicting plugins, or a theme that just has a bad bug.

Try switching to the BP default theme to see if the problem persists, so we can knock off the possibility that it is the theme.