Here is the Solution.
I have a spark plug part in SolidWorks.
When I render it, half of the body is inside the floor.
Now I close the render window and click Edit scene.
Change align Floor with to XZ plane which I assume is the floor.
Now i change the view to front view.
I drag the double arrow to downward which is the floor offset.
The floor is aligned with this arrow. Since this is below the model now, the floor will be just below the model.
I click OK and then final render.
Now the render is good and the floor is also OK.