To investigate the residual tensile stress formation mechanism of thinwalled piece in laser shock peening, the shock wave propagation law and the dynamic respond law of 0.5mm titanium alloy thinwalled piece are analyzed by ABAQUS software. The results show that when in reflecting in the thinwalled piece, the shock wave forms highintensity tensile wave and compression wave in turn. The stress distribution in depth is disordered under the action of tensile wave and compression wave, and is characterized by multiple peaks. The tensile stress field is located within 0.125mm beneath the surface and the residual tensile stress up to 426 MPa appears on the surface. The formation mechanism of residual tensile stress is revealed based on the shock wave reflection law. The maximum residual tensile stress is only 70 MPa in the 5mm thick plate and tensile stress at the surface translates into compressive stress by reducing the intensity of reflected tensile wave. Therefore, based on the principle of controlling the reflection wave pressure, a method of stress field regulation by exporting shock wave is proposed.