Abstract: Compared with the traditional power system, the time constant difference of the dynamic process of the new power system increases and the uncertainty of the external disturbance is enhanced. The time-domain simulation analysis based on the alternating solution faces the problem that the rigidity of the differential equations becomes stronger, which leads to poor convergence and reduced computational efficiency in the iterative solution process of the corresponding nonlinear algebraic equations. To this end, based on the fixed-point iteration theory, the iterative convergence mechanism of the alternating solution process of time-domain simulation is analyzed and the ideas to improve the convergence and computing efficiency of the timedomain simulation of the new power system are proposed. In view of the convergence deterioration factors of traditional alternating methods, an improved alternating solution method based on scalar Steffensen iteration is proposed, and the iterative calculation efficiency of the proposed method is analyzed. Finally, based on the IEEE standard 16-generator 68-bus system and an actual power grid with 2 970 buses, the effectiveness of the proposed method is verified.