The decarbonization of the building sector is crucial for global sustainable development because it is responsible for about 40% of global carbon emissions and has been growing continuously at a rate of 2% to 3% per year [1]. Over time, significant energy-saving milestones have been achieved through technologies such as improving equipment efficiency, building envelope performance, and utilization of sustainable resources. As energy consumption transitions shift towards low-carbon solutions with increased utilization of renewable energy, focusing on maximizing the use of renewables is pivotal in reducing building carbon emissions. However, wind and photovoltaic power have strong volatility and intermittency. Substantial evidence has supported the notion that adequate flexibility is necessary as renewable energy becomes predominant; otherwise, it may quadruple the comprehensive cost of energy use and even trigger energy insecurity [2]. Therefore, the building sector to adapt to an unstable energy supply through building loads flexibility and clusters' collaboration drive the further development of building decarbonizing.The concept of building energy flexibility refers to the ability to manage its demand and generation according to local climate conditions, user needs, and energy network requirements, according to International Energy Agency (IEA) Annex 67 project [3]. According to this definition, it is obvious that almost all electrical appliances in buildings can be subject to varying degrees of load adjustment through energy storage, frequency modulation, human behavior regulation, and delayed start. Besides, it is equally important to note that building components that do not directly consume electricity can still impact energy consumption and create energy flexibility [4], such as the building envelope, external shading, and curtains. Energy flexibility from buildings can play a critical role in the ongoing energy transition and hold tremendous shortterm regulatory value in future energy systems [5]. It is almost the most cost-effective method to match the actual energy generation and consumption [6]. The rational use of flexible building loads in a beneficial interaction with the power system is possible to decrease CO 2 emissions by 80 million tons annually before 2030 [7]. Figure 1 summarizes the regulation modes, sources, and evaluation indicators of building energy flexibility.