The primary time constant of power grids is the major factor of determining the decay rate of non-periodic components. Accurate calculation of the primary time constant is the important premise of transient characteristic analysis of current transformers. Based on the hybrid simulation function of the advanced digital power system simulator， an accurate primary time constant calculating method is proposed. By developing the electromechanical transient model of the whole system and electromagnetic transient model of the local grid， short-circuit current in different operating modes is obtained. After filtering， the non-periodic component of short-circuit current is obtained and hence the primary time constant. Simulation results show that the algorithm has fairly high accuracy.