The invention provides a method for preparing a bent car impact sandwich beam including a micro-truss structure. The method comprises a step of positioning a die so as to allow the die to contact with a bent bottom panel, so the die and the bottom panel define a holder which is filled with liquid photonasty polymer resin, wherein a mask is positioned on the holder, a set of UV light sources are arranged on an installation member relative to the mask, and the installation member is bent to adapt the shape of the bottom panel. Light from the UV light sources irradiate through holes in the mask so as to realize curing in the holder and formation of polymerized struts used for defining the micro-truss structure formed on the panel.