In this paper, a mathematical model of instantan eous positive sequence network is built through the instantaneous symmetrical component method. The model built for the 10 kV medium-voltage resonance grounding system, includes the variation of power source, petersen coil, ground capacitance and ground point. Based on this model, the mathematical expression of instantaneous positive sequence current real part and imaginary part in fault branch and unfault branch is derived. The character of the instantaneous positive sequence network is given as follows： in the steady-state after fault, the phase of instantaneous positive sequence current real part in fault branch lags behind unfault branch with a certain angle, while all unfault branches phase is almost equal; the amplitude of instantaneous positive sequence current real part in fault branch is bigger than the one before fault. The simulation finally validates the theoretical analysis.