The principle testing laser induced damage of optical thin films by laser photothemal deflection method (PTDM) and corresponding experimental testing set up are described. The laser induced damage on surface of optical thin films is tested by scanning surface using the set up. Grey image coded by tested two dimensional photothermal deflection signals is directly displayed and shown that the testing laser induced damage of optical thin films by PTDM possesses many advantages such as direct display of damage and determination of damage threshold. And the method is available.