If the offer is communicated by HR, you can negotiate through HR. You may make them a 'counter-offer' with the salary you want and see if they are open to it.

不是已经给你offer了吗？讲价钱最坏也就是维持原价吧，应该不会不给吧

我当时是他们让我先出价的，我听说应该要高一点，反正不会因为要的高就不给，大不了给得低一点他们认为合理的，结果没想到他们直接给我了

这工资多少到底谁说了算？

Usually big companies have their own policies in determining the saraly range for a new hire. For small company, however, it will be more flexible, and hiring manager may have more power.

会不会因为我讲价就不给我Offer了呢？

Don't worry too much, they have extended an offer to you, means that they are really interested in hiring you, and salary negotiaion is very normal.

Good luck.