The invention relates to an active power distribution network transient state real-time simulation system designing method based on an FPGA. The active power distribution network transient state real-time simulation system designing method can be implemented in an off-line simulation environment and an on-line simulation environment in a dividing mode, the off-line simulation environment is responsible for calculating the total clock period number-n and simulation using time-t calculated in each time step, the simulation step size deltat is set according to the real-time simulation using time-t, relevant parameters calculated according to the simulation step size deltat and the read basic parameter information are uploaded to the on-line simulation environment based on the FPGA. Real-time simulation calculation is completed in the on-line simulation environment, the simulation state is controlled through a finite-state machine, each time step includes the steps of historical item current source column vector calculation of all elements, total historical item current source column vector formation of the elements and linear equation system solving and updating of the elements, and the historical item current source column vector calculation step of the elements and the updating step are completely independent and can be processed in a concurrent mode. According to the active power distribution network transient state real-time simulation system designing method based on the FPGA, the real-time performance of the transient state simulation process of the whole system is guaranteed, and the good feasibility and the good applicability are achieved.