First, please forgive my poor English.

When I edit a xml/html file, the autocomplete’s popup window would not be shown， unless I press ctrl+space.

Is this a bug?

Add this to your User Preferences file.

"auto_complete_selector": "text - comment, source - comment"

Edit: So even though the autocomplete panel doesn’t show up, pressing tab will still commit your completions. You might like that it doesn’t show up after all.