Energy conservation at building’s operation stage is very important for the national economy. Building up reasonable methods to assess building operational energy consumption is critical and necessary. Japanese researchers use CEC (Coefficient of Energy Consumption), which is the ratio of actual energy consumption to ideal energy consumption, to assess building system’s operation level. In this paper, a concept of SCEC is proposed, which is calculated from building thermal simulation. Firstly, in order to build up a building thermal model whose input parameters are all from on-site measurement, the unmeasured parameters are obtained by backcalculation. Then, a building thermal model reflecting the reality accurately is built. Secondly, ideal energy consumption is simulated by the revised model with ideal indoor parameter replacement. Finally, SCEC is calculated. The method is applied to assess the operation level of the VAV system in Beijing Fortune Building. According to the case study, the feasibility of the simulation-based assessment method is validated.