I want to export it in 50i but it only export at 25p instead of the 50i. So i lose some fluid motion.
How can I proceed?
First, make sure all video footage is interpreted in the proper field order. Go File>Interpret>Main, and make sure it's upper field first. Add your comp to the AE render Queue. In the render settings, select upper field first, then render in the codec of your choice.
Are you using Final Cut Pro? It has a difficult time distinguishing between interlaced and progressive scan sometimes. Highlight the movie in the FCP Browser, and check the field order (incorrectly called Field Dominance in FCP) make sure it to is upper field first. Then check your edit timeline for the same thing.