Parallel FDTD method, by setting resistors as lossy media across grids, is used in time-domain simulation of large vertically polarized electromagnetic pulse(EMP) radiating-wave simulator with discrete resistors. Radiation fields in time-domain with different load-type of discrete resistors are given. Effect of different resistor load is shown. Electric fields in time domain of simulator with discrete resistors and a cylinder effector are presented. It shows that loading discrete resistors improve effectively waveform of radiated electric fields by reducing reflection from simulator's top. However, radiation performance may be weaken as simulators with resistors load. Energy entering into the effector through a square hole is much as the side with hole is near to simulator and parallel with polarized orientation of simulator. The method has general application, and can be used in simulation of time-domain fields of other large vertically polarized EMP radiating-wave simulators with discrete resistors, including simulation of coupling fields as the ground and effectors existing.