Abstract: With the increase of complexity of the grid and voltage level, the impact of geomagnetic storms on the power grid is also increasing. Calculating geomagnetically induced currents (GIC) generated by magnetic storms is critical to assess magnetic storm disasters and grid recovery after a disaster. The previous GIC calculations were based on the GIC-Benchmark model designed by the American Academy of Electric Power. This model emphasizes the calculation of the GIC and weakens the post-disaster grid recovery. This paper proposes an improved 24 node model in IEEE RTS-79 and uses this as a basis to calculate the geomagnetic induced currents of the branches in the grid under different degrees of magnetic storm disaster conditions, and to determine the weak links of the power grid in the environment of magnetic storms. The establishment of the model lays the foundation for the recovery of the post-disaster power grid in the future. This analysis method will play a role in analyzing the future geomagnetic storm's harm to the power grid.