Can i start seo for Under Construction Site??

My few web pages are underconstruction and i have started seo for my site, Can i get rank or should i complete my web first??

you've started SEO for your site, before you've finished it. Odd way to go about it but hey...

Whats normal is to build the site with basic SEO built into the template. Then when it goes live , monitor it, and then work on improving it for SEO.
You cant really do it the other way round.

Also an extra bit of advice for free. Ditch the 'under construction' pages. Have your index page outlining your services with some good information and contact details, and do your SEO on this until the rest of your site is ready to upload. That way, visitors wont get annoyed by seeing loads of dead pages, and you can start your SEO almost immediatley on your one existing page.

wrong wrong with that i did it to one of my domins without even having hosting for it

SEO processes begin before the site even goes live, before even the first page is coded, and continue at every stage from there.

I mostly build a site behind a "coming soon" page or a 3 or 4 page "mini-site" (but which always has content related to the real content that will follow).

I'd 'build in' seo as I go along, and be sure of one link to get the site indexed ASAP - but I would not start 'active' seo - such as link building - until the site was at least presentable. I hate attracting visitors who will then never come again.

yep i was talking onsite SEO, which you simply cant do (apart from sensible template coding) until the site (or at least part of it) goes live.

As for under constructions pages, they're well known to give your business a bad start. Better to build an informative one page site with good call to action. This is far better than giving little or zero information and risk losing or annoying your potential customers who you have misled to a site which isnt even active.

just go through search engine submission before your site is live.

just go through search engine submission before your site is live.

do not manually submit your site to search engines, get a link and let it be crawled

Submit to directories, not to search engines.