why may car died when I turn on the car air conditioner

What year, model, engine, fuel delivery system, and mileage on the car? Are you hearing any squealing sounds when the air conditioner is turned on?

Most air conditioning systems have an idle-up system when the air conditioning is engaged. Does the engine die when you turn on the air conditioner at cruising speed or just at idle?

The clues Ray O gave are worth considering. My first thought was that the engine is not set up right so is developing insufficient spare power to drive the a/c while at idle. (My Volvo 440 would keep the a/c off until about 10 sec after starting the engine, to let the engine reach proper running speed. But that car was not originally designed for a/c.) Check bits that go bad routinely: plugs/cables &c. And are enough fuel and oil getting through?