disappoint是及物動詞，一般後面不接介詞的。你說的應該是它的ed形式用法：

bedisappointedabout/with/atsth.

bedisappointedinsb.

disappoint是動詞原形，disappoint 為動詞，意思是讓……失望、令……失望

英英釋義：fail to meet the hopes or expectations of；to make sb feel sad because sth that they hope for or expect to happen doesn"t happen.

例句：

she says she doesn"t want to disappoint herself when she fails to keep them.

I hate to to disppoint you, but I am just not interested.

disappointed一方面是過去式可以當動詞使用，一方面是過去分詞可以當形容詞使用。disappointed 為形容詞時，失望的，沮喪的，失意的。

英英釋義：disappointingly unsuccessful;

例句:

why are you looking so disapointed about?

she was disppointed not to be chosen.

片語：disappointed at 對......失望

disappointment為名詞，失望