The influence of the surrounding buildings on the equivalent area of the considered building is analyzed in combination with the related standards by the Rolling Sphere Method. The method for calculating the equivalent area is described in detail and the calculating formula is given under three cases: the surrounding buildings being the same, higher, and lower, compared with the studied building, in the condition of the surrounding building in parallel with the studied building and ignoring the building’s width. Whereupon, the limitation of the formula in the standards is discussed. The research results could meet the demands of the more elaborate lightning protection technology.