Start the negotiation

"面试时，主管就问我是否接受比现有工作低的薪金。我说不可能，他坚持要我回答，我说Yes".

So you were willing to accept lower compensation but now changed your mind? You need to get your thought process straight first. If you didn't change your mind, then are you actually fine with lower compensation? Or you just weren't being honest with yourself during the interview?

Let's start with this. Between not getting this job and getting this job as is, which would you prefer? Sounds like you currently have a job so your choice is not between having no job and any job.

People have given many suggestions here on how to haggle over salary offers. You need to have plan A, B, C, and also ready to walk away from this offer. Think through, prepare yourself mentally, and call the manager to start the negotiation.

Don't blame the hiring manager, if you were him, would you offer a candidate more if you knew he/she's fine with lower pay? It's just a simple and plain business decision.