why does the game dragon age inquisition start downloading during gameplay

We're gonna need more information if you're expecting any help on this.

First off, what do you mean?
Are you on console or PC?
If console, what console?
Are you referring to the whole game, just the patch, DLC?
Where are you seeing that it's downloading while playing?

Is it downloading while you're playing DAI? Or while playing a different game?

Open up Origin and look at the settings. You likely have some things set to automatically update. I doubt it is the whole game downloading just as Ominous implied, it is more likely the patch or an Origin update. But you can turn off the automatic updates in settings, then just update when you want. It is a good idea to turn this off for games at least, so you can find out if the updates introduce bugs before patching.

Note this will cause Origin to say you cannot play games until they are updated, but if you set origin to offline mode you can still play the game.

i play it on ps4 console,it starts downloading after i finish tthe 1st quest after the intro

When you open up your downloads, what does it actually say for the title of the content downloading? Does it kick you out of the game?

As a workaround, basically you should be able to unplug from the internet while you are playing (or turn off your wireless settings on the PS4 before you start). The game should then not even try to check for available downloads, much less start downloading.

As for the problem itself, you need to provide more detail. It should not be downloading the game unless the first download was incomplete. But then I wouldn't expect you could even start the game if that was the case. Given you are just starting the game, you could be downloading patch 2, but it should ask you to download that before you even start (let you know an update is available).

So if you want to try to stop this, I would delete the game and download it again, then update it, download any DLC you have available, then try playing it, Hopefully that will stop the download issue. But you can use the workaround if you don't want to re-download the game. Hopefully after getting past the tutorial mission you won't have the problem, but I expect it will persist until you make sure all content and patches are downloaded.

Did you pause your download once the game reached playable?
Or did it hit playable, and you started playing?

If it's the first one, my guess is you've reached as far as the game has installed, thus, it would start downloading so you can keep playing.
If it reached playable and you just started playing at that point, the game downloads on its own in the background.