In situ investigations on the laser induced damage of the infrared windows by photothermal displacement technique (PDT) were described by the studies on the damage process of the Si window, Si window with half reflectancee protecting films and Si window with full reflectancee protecting films. It was found that the laser induced damage threshold of Si window can be highly increased by using dielectric protecting films,and the damage process of full reflectance protecting films is conducted layer by layer.