Hi Rong,
What's API and method parameters you're using to export DWF?
Note that Revit API provides DWFExportOptions class to customize the dwf export options, you may try to change the option to check the export.
Besides, we checked the document you provided, unfortunately however it is hard to us to find a clue how to recreate this problem. In order to diagnose, could you isolate your test project and send us a small, reproducible code sample? Note, please do not send us any information you think are confidential to your company. We appreciate your understanding and your patience again!