When warrant conversion is higher than mother price, company keep convert warrant is it good or bad

Not sure what you mean by company keep convert warrant ?!

After the warrant expired, it's worthless.

Only for strategic purpose will the sane people convert the premium trading warrants into mother shares - mother shares are too illiquid & any decision to scope up the mother share from the market will resulted the share price will fluctuate upward greatly.

If the above condition not present, there is no reason to convert the premium trading warrants. Instead, you can just buy from market the mother shares to increase the shareholding %.