A three-dimensional finite element model is established to perform the thermo-elastic-plastic analysis on stress and deformation of 2A12 aluminum alloy irradiated by lasers with different parameters in ANSYS software. The results show that with an assistant continuous wave (CW) laser to preheat the material, the effect of long pulsed laser efficiently increases with the increase of preheat time, materials yield time shortens, plastic deformation and yield zone augrrent. Irradiated by long pulsed laser with duty cycle of 10%, the temperature rise and deformation accretion of 2A12 aluminum alloy between pulses are not significant because of its high thermal conductivity. Increasing the preheat time is a more effective method than the power density increasing of CW laser to increase the plastic damage of material irradiated by the same long pulsed laser when their power densities are at 104 W/cm2 level.