No gas?

i hava mtd yardmachine, i got it to crank,but when i engage the blade it cuts off,could someone tell what that might be

It might be the seat switch.
Since you have not posted any model numbers, we have no idea which tractor you might have.
Find the MTD model number from under the seat, and the engine model numbers (and other numbers) from the engine itself. Post those numbers and we might be able to help you.

Also describe "HOW" it cuts off.
Does it struggle like a mower deck spindle is binding or like it totally lost spark?
IF the latter, it's likely a seat switch that is bad, mis adjusted or unplugged.