Under the new situation, there is an urgent need to model and simulate the power secondary system that highly shares information and implements real-time decision-making, in line with the modeling and simulation requirements for new power systems. In this paper, design methods are proposed to achieve simulation of power secondary systems by correlating the operating status of the power primary system. The smart substation secondary system with complex functional descriptions is taken as the research object. Firstly, an interrelated simulation method for power primary and secondary systems is proposed, and its simulation implementation framework, data interaction method, and data synchronization management are explained. This enables the actual electrical quantity data of the primary system to be transmitted to the secondary side, solving the problem of data source for the secondary system simulation; Secondly, a simulation design method for the power secondary system is presented in which the system-level interaction design, component-level class design, and module-level state design are proposed based on the object-oriented unified modeling language. Thus, the entire process of transmission, interaction, processing, and conversion of electrical quantity data in the secondary system can be analyzed; Finally, taking a short-circuit fault scenario at the 110kV side outlet of the 220/110/10kV main transformer bay as an example, and combining with the differential protection scheme, the effectiveness of the proposed simulation method is verified.