The invention discloses a power grid dynamic model and a construction method thereof. The model comprises a simulated generator set, a simulated transmission line, a simulated transformer bank, a simulated load and a forced zero sequence current distributor, wherein the simulated transmission line simulates distribution parameters of an original transmission line by adopting an equivalent chain circuit and piecewise concentrated parameters, the forced zero sequence current distributor is connected in series to the simulated transmission line and comprises a core unit and four phases of transmission lines, and the four phases of transmission lines are wound on the core unit and have equal wound number and same wound direction; and the forced zero sequence current distributor is used for enabling a zero sequence current value synthesized by three phases of current A, B and C to be equal to that measured on the N phase but opposite in direction. The constructed model can comprehensively and truly reflect various operating conditions of the original transmission line, and well solves the problems in complex power grid dynamic simulation experiments.