Abstract: The laser thermal stress induced in the aluminum alloy plate irradiated by CW/COIL was investigated by high temperature straingauge and thermocouple gauge and other diagnostic techniques in this paper. When the laser intensity irradiated on the target surface is about 1 000 W/cm2, the laser thermal stress descends rapidly with the increase of target thickness; for the target thickness h=1.00 mm irradiated by laser beams, and the laser intensity I=640～980 W/cm2, the laser thermal stress increases with the increase of laser intensity irradiated on the target surface. Both types of the laser thermal stress-time curves become complicated with the decrease of the target thickness or with the increase of laser intensity irradiated on the target surface. When target thickness h2.50 mm and laser intensity I800 W/cm2, the laser thermal stress strength is greater than the material fracture strength in the laser irradiation zone, holes and fracture strips occur and materials damage.