With the development of communication technology, the antenna of 5G base station is arranged near the main equipment area in the substation, which will inevitably cause electromagnetic interference to the secondary equipment in the substation. Therefore, the mechanism of electromagnetic interference from 5G base station antenna to secondary equipment in the substation analyzed in this paper, the mathematical model of radiation field of 5G base station antenna is established and its field source characterization formula deduced. Meanwhile, combined with the electromagnetic scattering theory, a mathematical model of the spatial scattering field of electrical large-scale metal equipment in substations under the radiation of 5G base station antennas is proposed. The field strength distribution experiment test of 5G antenna in Henan 500 kV Guandu Substation verifies the accuracy of the algorithm. Finally, the influence of different operating frequencies, transmission power and downtilt angles of 5G base station antennas on the electric field strength at the secondary equipment is studied. The calculation results show that the base station antenna transmitting power and downtilt angle significantly change the electric field strength at the secondary equipment, and the maximum can reach 12.92 V/m, exceeding the electromagnetic compatibility immunity threshold.