He is disguising himself as an old man for the costume party.

disguising 是动词 disguise 的现在分词形式。disguise 由前缀 dis-（表示"否定、相反"）和 guise（"外表、伪装"）组成，意思是"伪装、假扮"。 disguising himself as... 表示"把自己伪装成..."。