Shall i accept the new position in the same company to be a training instructor instead of product support engineer, is it useful for my career?

it is dependent on u are u have able to learn and can do then new task

and in any way this is good in ur career

First of all you have to ask yourself if you will like the change to the new duties and then think for the rest like salary, career etc 

Thanks

Agree with Colleague Ahmed Mohammad answer

Only you can tell, but it probably is.  New skills and experiences are almost always valuable. 

Yes. It will widen your knowledge.

This totally depends on whether you believe you want to be a trainer for the next couple of years! Once you take up this profile, it might take some time before you get back to your technical line of expertise since training becomes a HR role. So please consider whether your career ambition is to be a techincal trainer or a techincian yourself. This will be a more passive role compared to being an Engineer hence please consider on those basis especially since salary is the same. You need to check with your managers on how your new growth chart looks like since that will show you where you will be few years from now.

Good luck!