The invention discloses an electromagnetic immunity testing system building method, which comprises a measurement system, a control system, two pulse high voltage sources and a bounded waveguide transmission line connected in sequence. The testing system building method comprises steps: 1, according to long pulse and short pulse in a theoretical electromagnetic pulse radiation field, technical parameters of a transient electromagnetic field are determined respectively; 2, according to the technical parameters, parameter setting on the bounded waveguide transmission line and the two pulse high voltage sources is carried out; 3, the two pulse high voltage sources are respectively connected with the bounded waveguide transmission line; and 4, whether deviation of technical parameters of an actual electromagnetic pulse radiation field meets design standards is verified. The service life and the environmental adaptability of a tested device can be improved, the cost is saved, the design accuracy is high, the actual system parameter reference value is large, various experimental environments can be configured flexibly, and practicability is strong.