The invention discloses an outdoor pre-installed transformer substation and an installation method thereof. The transformer substation comprises a main box body and a top cover, wherein the top cover is fixed on the top of the main box body and comprises a modular end sealing plate and/or a modular seal plate I and a modular sealing plate II, one end of the modular end sealing plate is of an inverted L-shaped bent structure, two ends of the modular sealing plate I are both of the inverted L-shaped bent structures, two ends of the modular sealing plate II are bent into a convex structure, and two ends of the modular sealing plate II are respectively fixed with the modular end sealing plate and the modular sealing plate I, so that the convex structure of the modular sealing plate II covers the outer surface of one inverted L-shaped bent structure of the modular end sealing plate and the modular sealing plate I. The installation method comprises a plurality steps such as bending and rolling molding manufacturing, hoisting laying and fixed assembling. The invention effectively overcomes the technical defects of high manufacture cost, large weight, complex structure, long manufacture period, liability to integral deformation, and difficulty in mounting, dismounting and maintenance of the traditional top cover.