How to convert solidworks assembly file into .obj formate ?

You're going to need a third party add-in for SolidWorks, or use a "middle-man" software that can export whatever SolidWorks can output into an OBJ. There is no way to do this in SolidWorks natively.

Save your assembly in step ot iges formats.
Download Freecad
https://www.freecadweb.org/downloads.php
when using freecad, open new file, import step or iges
Then you can export to obj or stl.

Alternatively you can save the solidworks assembly in stl format and them use freecad to export in obj.