That night,as she lay in bed, she thought about the money. 請問一下這裡的as she lay in bed是否為形容詞子句的非限定用法,如不是的話可否告知為何要這樣寫,謝謝?

這as she lay in bed不是「形容詞子句」，而是「副詞子句」，提供的是與she thought同時發生的另一個行為。因為移到主要子句的前面，所以加上逗號表示附屬子句「越位」，跑到主要子句的前面了（通常副詞子句的位置放在句尾，不加逗號）。

這個「副詞子句」修飾的是主要子句的「述部」：thought about the money，詞性是「副詞」。

「形容詞子句」修飾的對象是在它前面的「名詞」，所以詞性屬於「形容詞」。

That night,as she lay in bed, she thought about the money. (X)
That night, as she laid in bed, she thought about money. (O)
Laying in bed, she thought about money that night. (O)