Thank you for your advise.
May be I have wrongly setup the Reading Settings in the admin panel. As you know, I have a page with many numbers of sub-pages and sub-sub-pages containing posts in it. So for my Reading Settings under a static page, the front page is set “Home”, then the posts page I leave it “– Select –“, is this the correct way ?
As what I know, most of the excerpt plugins require the posts page under Reading Settings to be set.