When you drive a car and try to stop or reduce speed by braking sometimes the car stops and switches off your idling goes so low. I had recently changed timing belt. can it affect this?

If you car stalls when you remove your foot from the accelerator, when you brake for a stop, you will find that you have a vaccum leak, in one of the hoses from around the engine. A vaccum leak causes an engine to run rough, stall and give poor acceleration. Any hose that you can see with cracking needs to be replaced.