Why sometime pleinty white smoke came out of exaust

White Smoke...
1. Typically normal condensation in the exhaust system converting to steam once engine is running and producing heat creates white smoke. Typically this lasts only a short period of time until condensation water in the exhaust system is gone.

2. If there is a problem, persistent white exhaust could be an indication of a water leak into the cylinders (e.g. cracked head or leaking head gasket. The good news is, this second case will normally will occur with other problems (e.g. consistent rough idle, or rough engine vibration at specific RPMs). If this is suspect, keep an eye on the oil with the dip stick and during next oil change (If any of the oil appears WHITE, there is a problem.). If this is a suspect problem, a test for the DIY'r mechanic would be to remove all the spark plugs, have another person turn the engine over, and watch for water mist to spray out of one or more cylinders.

Because you said sometimes, this sounds like normal condition.