The shape coefficient of building (SCB) is a measure of correlating a building’s shape and its energy consumption. However, strictly adhering to restrictions the SCB may not be conducive to reducing energy consumption, as it disregards the potential benefits of solar radiation. To address this, a novel concept and method for determining the modified shape coefficient of building (MSCB) have been introduced in considerations of heat loss and solar heat gain. Field measurements were conducted in western Sichuan to establish a comprehensive understanding of the factors influencing indoor thermal environments, involving collecting indoor air temperature data for model buildings under natural and heating conditions, including solar radiation, building orientations, and U-values. Results demonstrated that maintaining an identical SCB did not consistently ensure similar indoor thermal environments. The indoor thermal environments were significantly influenced by building orientations, U-values, and solar radiation, highlighting the limitations of relying solely on SCB. A simulation method validated through experiments was utilized to predict the dynamic heating load of an actual building. The influence of building orientations on the MSCB across different time scales and actual climatic conditions was investigated. Results illustrated a potential reduction of the SCB by up to 32.4% in the specific case. This reduction correlated with trends observed in the monthly correction factors and heat consumption, suggesting that moderate relaxation of SCB restrictions could be viable even in scenarios with high heating demands. This research contributes valuable insights into the realm of energy-efficient building design.