The invention discloses a preparation process for an ultrathin prefabricated thermal-insulation external wallboard used for building industrialization. The preparation process comprises the following steps: pouring a first layer of RPC materials in a mould; then, placing one layer of thermal insulation materials on the upper surface of the first layer of the RPC materials; and finally, casting a second layer of RPC materials on the thermal insulation materials, maintaining and curing to obtain a RPC ultrathin sandwich wallboard. According to the preparation process, RPC concrete is adopted to carry out lightweight design on a sandwich wallboard, so that in case of meeting load, the weight of the wallboard can be reduced, the durability and the mounting convenience of the wallboard are improved, and therefore, an ultrathin assembly type prefabricated external wallboard which is integrated with functions of load-bearing, light weight, thermal insulation, water resistance, fire resistance and the like is provided. The external wallboard has the advantages of being good in water resistance and durability, not liable to generate micro cracks and the like because of a compact tissue structure. A thin wallboard, with a thickness of 9-12 cm, produced at present can arbitrarily design a required section shape in addition to reducing dead load of a component, further can be applied to an assembly type external wall, a curved-surface decorative external wall, and the like of building industrialization.