The thermal stress model was put forward in the paint removal from substrate by Q-switched laser pulses. With the one-dimensional equation of heat conduction, the thermal stress generated by thermal expansion was calculated. The adhesion force between the paint and the substrate was calculated according to adhesion formula. If the thermal stress is greater than the adhesion force, the paint will be removed from the substrate by the laser action. When the Nd∶YAG laser with wavelength of 1064 nm and pulse width of 10 ns is used to remove orange paint from Fe substrate, the cleaning threshold is 0.5 J/cm2 and the irradiation time required for paint removal and the temperature change of Fe substrate were calculated according to the theoretical model. The experimental cleaning threshold is 0.57 J/cm2, which is in accordance with the theoretical result. From the experiments and theoretical model it is found that if the laser fluence is too high, the temperature of Fe substrate will exceed the melting point of Fe and the Fe substrate will be broken. And the paint removal on Al substrate by laser was taken into account.