get your hopes up把希望抬高--表示期望过高，don't get your hopes up就是别抱太大期望。

【例句】
I'll do what I can, but don't get your hopes up.
我会尽力而为的，但别抱太大希望。

Don't get your hopes up. There'll be a lot of people applying for the job.
不要抱太大希望，应聘这份工作的人会很多。