The area inside the property boundary of a plot is often considered privately owned. However, resulting from the complexity of urban form transformation and urban planning regulations, the area within a plot boundary can also have public or semi-public attribution. The accessibility of the internal area of a plot is defined as the ‘plot openness’ in this paper. The openness of plots affects the spatial continuity in a city and reflects the mode of land development and urban planning. At present, the evaluation and analysis of urban public space have always been a hot research direction. But the openness of plots, the basic unit of urban forms, and its influencing factors have seldom been considered. This research aims at presenting a quantitative assessment framework to measure plot openness, and explore the potential factors including built density (FAR) and construction time that may affect the transformation of China’s urban plot openness. Taking Nanjing’s downtown as a case, this paper employs mapping to present the openness condition of the selected plots based on access and control type. Then, it proposes indicators including public space density (PSD), quasi-public space density (QSD), and depth of plot space (D) to quantitatively assess plot openness. At last, the regression analysis result reveals no strong linear relationship between plot openness and corresponding built density (FAR) in Nanjing’s downtown, whilst the descriptive statistic base on the development period shows that the transformation of plot openness is indirectly influenced by the land-use policy and urban planning regulations enacted in different periods.