Laser peening forming is a novel technology, which employes the residual stress induced by high-power repetition laser to form plate. The mechanism of laser peening forming under elastic pre-loading condition was analyzed and the feature of deformation was tested by experiments. In the experiments, the parameters of laser were as follows: the wavelength is 1.054 μm, the pulse duration is 23 ns, and the power dsnsity is 109 W/cm2. The material of sample was LY12CZ aeronautic aluminum alloy. The experimetal results show that the flexural moment of the sample in the prebending direction is far bigger than that in the vertical direction to pre-bending and the spherical shape can be conquered during pre-loading laser peening forming to a great extent. Under the same conditions, curvature of workpiece after the pre-loading laser peening is two to three times than that after laser peening without pre-loading, and the surface roughness can be improved by one to two grade. So the laser peening forming during pre-loading is superior to the laser peening without pre-loading.