Hi Anis,

>The problem is that the bottom surface is come from multiple curves ( check my original file on my first post ) :

Yes, but your original surfaces/curves are not good for later blending/constructions. Even in Rhino a 2 rail sweep using your curves will not give a clean/smooth surface.

Rhino:- 2 rail sweep:-



The problem is that the curves for the main body are not all connected to at least G1, so it will cause further problems, even possible problems with output for rendering. It is better for such a simple shape to use a circle or oval and then move the control points to create the correct shape as I did on the examples I posted.