应该是：
on the first(或者 1st) floor
on the first floor
在一楼
例句:
1.
We can draw pictures in the art room on the first floor and read story-books in the library on thesecond floor.
我们可以画画的美术室一楼和阅读故事书在图书馆的二楼。
2.
The old lady lives on the first floor.
那老太太在一楼住。
