Under the new situation, there is an urgent need to model and simulate the power secondary system which highly shares information and implements real-time decision-making, in line with the modeling and simulation requirements of new power systems. In this paper, design methods are proposed for the first time 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. First, 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, which enables the actual electrical quantity data of the primary system to be transmitted to the secondary side, solving the problem of data source in the secondary system simulation. Then, a simulation design method for the power secondary system is proposed, incorporating system-level interaction design, component-level class design, and module-level state design based on the object-oriented unified modeling language (UML). Thus, the entire process of transmission, interaction, processing, and conversion of electrical quantity data in the secondary system can be analyzed. Finally, to validate the effectiveness of the proposed method, a case study is conducted using a short-circuit fault scenario at the 110 kV side outlet of the 220/110/10 kV main transformer bay, in conjunction with a differential protection scheme.