i have used this :

{
    "mob": "minecraft:wither",
    "result": "default",
    "healthadd": 300   
   }

but i haven't changes, suggestion?

I wrote it in the spawn.json

The problem is most likely that the wither doesn't spawn naturally. It spawns using some specific sequence of operations and that bypasses the normal spawn rules. You could try if it works if you add the 'onJoin' true rule like this:

"onjoin": true,

Let me know if that helps

Okay, i try now, sorry if I answer you just now, I've been very busy lately

it works!!!! Now the question is, does it works for dragon too?

Other question, how can i add the attack damage to the wither/enderdragon boss?
It doesn't work with damageadd

My code : { "mob": "minecraft:wither", "result": "default", "onjoin": true, "healthadd": 400, "damageadd": 400 }

it works!!!! Now the question is, does it works for dragon too?

it doesn't works for enderdragon :'(