This paper presents the design of InterPSS simulation engine, including its object model, open software architecture, and software development process. Several advanced applications, including an integrated transmission and distribution co-simulation, an electromagnetic transient and phasor domain hybrid simulation, and InterPSS integration with a market simulator, have been developed by either extending InterPSS simulation engine or integrating it with other programs and/or platforms. These advanced applications show that the open architecture combined with the comprehensive modeling and simulation capabilities make InterPSS a very attractive option for the research and the future new power system simulation application development.