Building energy consumption accounts for around 25% of China's annual energy consumption. Under the background of fully implementing the goal of " carbon peak and carbon neutrality", developing low-carbon buildings is a promising approach for the construction industry to realize the goal of "double carbon". Meanwhile, district energy, which can effectively improve the share of renewable energy in the energy mix, cut the energy cost and reduce environment impact, has received increased attention. Building a district energy system nearby existing buildings can effectively eliminate building carbon emissions and help achieve the goal of "double carbon". This paper reviews the efficient utilization of district energy in buildings. Firstly, it discusses the components and functions of the district energy systems, and then expounds how to improve the system performance from four perspectives: Source, Network, Load, and Storage. It was observed that the technologies of multi-energy complementary, demand response and energy storage based on optimization algorithms can be applied for peak shaving and promoting the share of renewable energy, thus reducing building carbon emissions. Finally, the study discussed the current status and future trends of district energy system in China. The paper concludes that a digital twin city which is capable of perception, diagnosis, prediction and intelligent control will be the direction future cities.