At present, relatively few tools are able to visualize regional fire risks both dynamically and efficiently. As a general method of density clustering and visualization, heat maps can be used to analyze the spatial distribution characteristics of fire risks. In this study, based on combining a transformed sigmoid function with a heat map method, a transformed sigmoid risk model (TSRM) was proposed. A numerical mapping method between fire risk values and colors was established based on an RGB color model. By using HTML 5 and JavaScript, a fire risk visualization tool (FRVT) was developed based on the TSRM. Baidu Map was used as the geographic information system engine for the FRVT by calling its JavaScript application program interface. To validate the TSRM and FRVT with case studies, 16,448 historical fires from 2013 to 2017 in Changsha, China, were gathered and visualized. The visualization results indicated that the high-risk areas were mainly distributed in urban areas, and that the medium-risk and low-risk areas were primarily distributed in rural areas. The FRVT can also be applied to visualize and analyze process-related fire risks from a regional perspective. In further research, it will be important to update the FRVT, e.g., to visualize the fire risks posed by the transportation of hazardous materials.