回答如下：

以上句子语法结构为：

主语：Dad 爸爸

谓语：dressed up as 打扮成

宾语：Santa Claus 圣诞老人

特别备注(1)：dress up as 中文意思：装扮成/打扮成

特别备注(2)：可以直接把dress up as看作为一个短语动词作句子谓语

特别注意：主语补足语可以看成是宾语补足语的被动语态形式.

例如：

Tom often makes his younger sister cry .（宾语补足语）

His younger sister is often made to cry by Tom.（主语补足语）

Tom made his mother angry this morning（宾语补足语）

His mother was made angry this monring by Tom（主语补足语）

dress up as作动词词组理解感觉有些牵强，因为词典里都没有这个词条。我觉得像状语，as短语作状语修饰dress up。

继续为你解答

【1】dress up as 可以当作整体看成一个短语动词

Little girls dress up as angels for fiestas.

小女孩们在节日里扮成天使的样子

【2】dress up打扮；穿上特殊服装；（使）装扮成另一种样子；为取乐和假扮而穿别人的衣服)

(1)装扮;打扮

You do not need to dress up for dinner.

你们不必特意盛装赴宴。

I just love the fun of dressing up in another era's clothing.

我就是喜欢穿另一个时代的衣服玩。

Little girls dress up as angels for fiestas.

小女孩们在节日里扮成天使的样子

(2)给…打扮;给…装扮

Mother loved to dress me up.

妈妈喜欢打扮我。

(3)修饰;美化;粉饰

Politicians dress up their ruthless ambition as a pursuit of the public good.

政客们将他们的狼子野心美化成是谋求公共利益。

However you dress it up, a bank only exists to lend money.

无论你如何粉饰,银行就是为了放贷而生的。

Dad dressed up as Santa Claus.

划线部分是状语。