More than half of the world’s population lives in urban areas and this figure is expected to increase. The worldwide trend is in the direction of urbanization. Building sustainable cities is one of the sustainable development goals (SDGs) initiated by United Nations Sustainable Development Solutions Network. In the anthropocene of human induced climate change, what makes a city sustainable? This paper takes Beijing as the case study, uses building smart infrastructures and lowering ecological footprints as the main thread, uses a mixed research method of questionnaires, interviews and site survey, and discusses four aspects -- transportation, waste management, green spaces and energy saving buildings -- from employees’ perspectives of how to a build a sustainable city. This paper also discusses the relations between smart infrastructure and the resilience of a city, and relations between lowering ecological footprints and a green city, and draws a conclusion that a resilient and green city is a sustainable city. We can build a sustainable Beijing only through vigorously developed public transportation, well managed waste, the development of diversified green spaces, and buildings that efficiently use natural resources as energy supplies.