how to disable auto smoothing by default in 2.74 when I import an obj file, by default auto smoothing is enabled. Is it possible to change this option as in the old realises?
You cannot (well, not from UI or other 'user-land' ways). Note however that 'autosmooth' here is a bit of a weak name, since it actually means you are using (custom) normals as defined by the .obj, instead of auto-computed ones by Blender…

Now, if you really want to disable that by default, you'll have to open addons/io_scene_obj/import.py in your Blender installation, search for me.use_auto_smooth = True (should be around line 657 currently), and replace it by me.use_auto_smooth = False (or just comment it by adding a # in front of that line).

But again, don’t think you want to do that, at least I do not see why you would…