Digital Twin can establish an accurate correspondence between actual objects and digital models, so that digital analysis can be performed based on data to guide the improvement of the objects. Taking the civil engineering building at University of Science and Technology Beijing as an example, the fire safety assessment of the building and retrofit was implemented on the basis of digital twin framework. Firstly, Building Information Modeling (BIM), Internet of Things (IoT), and Cloud technology are employed to collect and integrate the real-time sensing data (e.g., wind speed and number of people) and thus establish a digital twin model of the civil building. Secondly, the simulations on fire and evacuation by using sensing data are performed under the digital twin model, and results processed by traditional estimated data are compared. Finally, the paper proposes the retrofit plans of building fire safety and retrofit effects are tested according to the results above, which provides accurate references for building fire safety retrofit. Moreover, the results in the paper note that the results of the fire safety assessment by using the digital twin framework are closer to reality, hence it can guide building fire safety retrofit.