I think it wise of him not to take his advice.怎么不用don't否定

it is +adj. of (for) sb. （not）to do sth. 是一个固定句型。还有正确句子应该是：I think it is wise of him not to take his advice.

这里it是形式宾语，不是一个完整的句子，所以用了动词不定式的否定结构：not to take。
it如果是形式主语的话，应该是I don't think it is wise of him to take his advice.这样就用don't了。

don't的话意思变了 成了 我认为接受建议是不明智的 而原句是我觉得他不接受建议是明智的

Don't 用于祈使句句首。