The Integrated Energy System (IES), as a platform for the coordinated optimization of various heterogeneous energy sources and the complementary use of multiple energy forms, provides an effective solution for energy conservation and emission reduction while improving energy efficiency. However, the high integration of renewable energy and the coupling of diverse energy loads present significant challenges for system scheduling, placing higher demands on the flexibility of IES systems. Additionally, with the steady advancement of power market reforms, a multi-entity participation model in optimization has emerged, including IES operators, resulting in a diversified competitive landscape. This thesis focuses on system flexibility, exploring various flexibility resources within the IES, and establishes a low-carbon economic scheduling model for IES that takes into account both flexibility and multi-entity participation. Through coordinated source-load optimization scheduling, it balances economic efficiency and system flexibility at the system level, harmonizes the interests of multiple parties at the entity level, and reduces system carbon emissions in terms of environmental benefits.