The invention relates to a preparation method of a prefabricated moulding bed, which is widely applied to steel lining prefabrication construction in a nuclear power station particularly and belongs to the technical field of construction of the nuclear power station. The preparation method comprises the following steps of, firstly, welding base H-shaped steel into a meshy block structure; weldingan upright post on the meshy block structure; respectively welding longitudinal arc angle steel with a longitudinal upright post support, and welding traverse bus angle steel with a traverse upright post support respectively; and connecting the two upright post supports mutually to form a support structure. The process can effectively decrease the integral welding deflection, and is favorable forguaranteeing the integral stability of a construction member. The moulding bed structure prepared according to the steps can completely achieve the prefabricating precision of a nuclear island steel lining, meet the strict requirements on the level 1 nuclear quality and the nuclear safety, and guarantee the stability of production quality.