The invention discloses a design method for a ventilation opening of a preassembled transformer substation assisted by a computer, which includes the following steps: mounting Icepak simulation calculation standard software on the computer; setting a transformer as a heating source; building the 3D stereoscopic model diagram of an equivalent preassembled transformer substation; setting the thermophysical parameters of a box body and an inside clapboard in the Icepak simulation calculation standard software; building a transformer model on the 3D stereoscopic model diagram according to the capacity of the transformer; repeatedly calculating the indoor temperature of the transformer according to the thermophysical parameters of the box body and the inside clapboard, the capacity of the transformer, an air outlet and an additionally arranged air inlet as well as correcting the area of the air inlet according to the temperature so as to lead the temperature to achieve the design requirement. The design method has the advantages of having a fast design speed, calculating accurately, reducing the working intensity, reducing the design cost and improving the success rate of the design scheme; besides, the invention can design a preassembled transformer substation which has the largest natural ventilation area, the best ventilation opening and more reasonable arrangement of the insideapparatuses.