For example, the scale of annotation drawing is 5:1, in Creo Elements/Direct Modeling Annotation the dimension text of line is 100, after export to dxf file and open it with AutoCAD, then measure the length of line, the dimension text is 500, not 100. how to keep the dimension text as Annotation when measure length in AutoCAD again.
This is not that simple. The top part of the Annotation drawing is always 1:1, the subparts (sheets) also 1:1.
The scale is attached to the views, in your example 5:1.
Since Autocad doesn't have this functionality, the scale is evaluated according the top part scale so 1:1 and the geometry is scaled to look the same.
Manually, the only way is to change the view scales to 1:1 and then generate the DXF.
Of course there is LISP and macro programming possible to automate this. In the past, we have written some tools to output a view to 1:1 DXF for laser purposes for instance.