I open a assembly file with step format .
When open file I can not access the part in feature design tree
How can I fix this problem .
You can switch off "3D Interconnect" and will not have that issue in the future.
Or you can do it as Daniel Mapp suggested.
Right click on the top level in the feature tree and select dissolve feature:
This will break the link to the original STEP file and should allow you to open the 3D sub parts.
Retrieving data ...