Let her know what your mind.

let her know what is on your mind.
let her know your thoughts.
let her know your mind.

不对，应该是Let her know what is your mind.

Let her know what your idea is.

Let her know what you think.

what后面跟一个从句，应该接一个完整的句子。your mind只是一个主语。

Let her know what is your mind
让她知道你（的）想法

不对
Let her know what's in your mind.
我觉得应该是这样。