Thanks for your answer.
after getting ur answer i have notice that my pc not gets shut down but it turns off.
So please reply the answer
Is there is the problem in SMPS or.....?

Thanking you

You are supposed to continue where you started first, please don't start new topic for same question.

Anyways..
There can be many reason..
1. Heat problem
2. Wiring problem
3. Some hardware issue.

For first one.. download software named "Speedfan" and see what temperature is there inside your pc, if its exceptionally high then you have to work upon it.

For second, I had same problem once, there were sparks inside electric socket, though it looks firm from outside and had hard time to find the problem as was not visible.

For third one, if above two is not the problem, we need to check third.