Yes, thus is commonly used as a greeting when you haven't seen someone for quite some time. It's not really a sentence, but it is used like this.

'Is the sentence "Long time no see" really used in English?' Yes, it is. Native speakers do use this phrase. Of course it doesn't follow the rules of grammar.We know this! We know that it is 'pidgin English', and that's why we use it. It's short, simple, and deliberately strange. As with some advertising slogans, popular catchphrases and quirky song lyrics, sometimes it's fun to say things which aren't quite right and don't follow the rules.

This is a Chineselike expression.because more and more Chinese people use that,which makes Westerners gradually accept it.so native speakers also use it.and think it is correct.

Yes, it is. It's some kind of catchphrase, a pattern of words that becomes popular because people like something about the way it sounds. It does sound like it might be an imitation of a stereotyped Chinese-American speech pattern. As usual, Wikipedia has a good article about it; I can't add anything to what Wikipedia says: https://en.wikipedia.org/wiki/Long_time_no_see