Why my iPad Air shut down by self ,after change battery still same

A bit more detail on the symptoms would help but based on what you say, the iPad shuts down by itself. Normally, that would most likely be a battery issue. If the replacement battery does the same thing, it could still be a battery issue so you will need to isolate a variable here. Ideally, you need access to known-good parts to properly troubleshoot an iPad.

In the absence of that, if you can measure the voltage of the battery directly with a multimeter, that could help us determine if the battery is charge or not. A fully charged battery would measure ~4.2V and a depleted battery would be around 3 - 3.2V.

If it is charged, then the problem would be on the logic board.