The invention discloses a prefabricated safety device for a secondary protection screen of an ultra-high-voltage transformer substation. The prefabricated safety device is composed of a protective cover, a cloth roller shutter, a lower rod, a safety lock, and a safety lock buckle. One side of the cloth roller shutter is fixed in the protective cover and the other side is fixed at the lower rod; and the buckle is arranged at the lower rod. With the device disclosed by the invention, occurrence of a secondary substation accident is avoided effectively; the safety measure units are arranged at the front and back of the protection cabinet, so that the safety of the working environment is improved; and the device has a simple and effective structure and high practicability. When the operation and maintenance personnel makes corresponding safety measures, back-and-forth movement of the red/green cloth curtains among all relay protection chambers is not needed, so that the work efficiency isimproved substantially. The prefabricated safety device can be applied to transformer substations at other voltage levels.