This study develops a new perspective of analyzing urban heating system (UHS) by taking a looking at the heat transportation flexibility of the system. We proposed the heating system flexibility (HSF) concept to represent UHS’s capability of delivering heat to the substations (and the buildings) under different operation scenarios of valves and pumps. We also give out the corresponding evaluation method. Based on the proposed ideal and its quantification method, we take a real UHS as an example to show the effectiveness of HSF by investigating the impact of heating network topological structure change. When a topological change close to end user is introduced, the average flexibility increases but the median flexibility drops. We find that system-wide topological structure change such as new pipeline does not solve the imbalance of HSF among substations. Overall, the concept of HSF has the potential of being used as an important criterion in the design, operation, and control of future large-scale complex multi-source UHS.