In the post-COVID-19 era, it is necessary for cities to increase their level of resilience to cope with unexpected risks. How can they improve their level of resilience? This article will stress research on the most vulnerable units in cities and further identify key vulnerability indicators of these units. The study area is in Wuhan, China, and the research object is the old community. In China, urban governance generally follows the "City -District -Street -Community" model, the "street" is the city's cell, and the community is the basic unit of urban functioning. Meanwhile, grassroots (community) governance is divided into a more fine-grained pattern: "Community -Subcommunity -Community grid -Building." Unlike the internationally accepted term "slum," the old community is a specific concept in China. It usually refers to communities with many vulnerable people living in, whose functional decline, structural dilapidation, environmental deterioration, because of the long period since its construction. So, the old community is the most vulnerable part of the community, and its level determines the upper limit of the city's resilience (Cannikin Law). First, this paper systematically reviews the development of Chinese communities after 1949. It classifies old communities into three categories: the street committee system, the danwei system, and the community-based system. Second, three dimensions of resilience: physical environmental, social environmental, and individual consciousness dimensions, were derived by examining internationally sophisticated resilience assessment methods. Third, the paper observes old communities in Wuhan under these three dimensions, elaborating a series of vulnerability indicators and find out reasons. Besides, data were collected from participant observation, archives, interviews, newspapers, and published reports. This paper aims to provide a reference on how to fundamentally improve cities' resilience levels, namely, to start with their basic units and address the weakest parts.