Sir i taken admission in wbjee then i sit on mop up round then i got another college . My previous college seat will cancel??

Yes if you register for mop up round it means you are willingly losing the seat you got during the rounds pf counseling so please decide carefully that whether it is worth that risk or not because the chances of getting a seat in mop up round depends upon your marks,category and also subjected to the availability of seat in the institutes.
Good Luck