Abstract: For the 2.5-dimensional simulation of the whole system from HPM(high power microwave) source to antennas’ radiation, a scheme of finite-difference time-domain(FDTD) near-to-far-field transformation in 2.5-dimensional simulation is proposed based on the simulation of near fields. The closed three-dimensional column surface is got through rotating the output-boundary in the 2.5-dimensional simulation domain about the symmetry axis. The equivalent electric and magnetic currents can be obtained from FDTD near fields on the closed surface with equivalence principle. The far-fields radiated by these currents are then calculated using the radiation field expressions. The examples about far fields of conical-horns excited by circular waveguide TM01 mode or TE01 mode are given. And the results agr