This study attempts to take the evolution of the theory of medium specificity in painting as the background, and re-examine the 'flatness' proposed by critic Clement Greenberg, i.e. the irreplaceable medium specificity of painting as an artistic medium. Scholars have long seen flatness as a lazy and negative definition of singularity, obscuring the diverse possibilities of painting. Whereas this study will challenge that argument, we argue that as the theory of medium specificity in painting has gone through the empirical critique of modernism, the deconstructive reflection of postmodernism, and the meta-modernist mediation of the theoretical contradictions and strengths of the previous two periods, flatness is no longer a singularized definition but rather a composite theoretical framework that embraces the universal perceptions, practical experiences, and conceptual cognition of the painting. The framework not only illustrates the diversity of painting arts, but also continues to effectively mark the unique position of painting as an artistic medium within the complex context of contemporary art.