The focus of this paper is the electromagnetic characteristics of cylindrical wire grid cage, an equivalent substitute for a hollow conductor cylinder. First of all, the equivalent radius is derived based on the conformal transformation and the equivalent electromagnetic field theory, respectively. Then, the electromagnetic simulation software CST is used to simulate the influence of the density and thickness of the metal wire grid on the risetime, pulse width and peak value of the electric field in the test area. Further, the electric field distributions of the cylindrical wire cage and the hollow conductor cylinder are compared. The results show that the cage structure can completely replace the hollow conductor cylinder and verify the rationality of the equivalent radius theory.