The damping effect in grid forming (GFM) control is inherently large and thus has great impact on its transient stability. Although qualitative understanding of damping has been developed in recent studies, an analytical and intuitive method for quantitative assessment of damping is still needed to be developed to date. This article visualizes the transient damping energy through the concept of damping area on a tailored phase portrait to physically and intuitively understand the damping effect. On top of that, the damping energy approximation (DEA) method is further proposed to depict transient stability boundary more accurately and evaluate the critical clearing angle analytically. Moreover, the conservativeness of proposed DEA method for GFM converter with both constant terminal voltage control and reactive power-voltage droop control is analyzed in theory and verified in time-domain simulations and control hardware-in-loop experiments. This method quantitatively and visually evaluates the impact of damping on transient stability boundary and critical clearing angle, which avoids complex numerical iterations and may provide some insights into the transient stability analysis of GFM converter.