This paper presents a diagnostic assessment of fire safety by using the extension engineering method, which can also be applicable for all kinds of buildings. Firstly, we select one building as the object and formulate the main direction of fire safety assessment in accordance with its characteristics. The assessment is divided into sub-items, which are weighted by importance. And then, design the evaluation program by using the extension engineering method and enter the actual parameter values acquired from the object site to obtain the results of assessment. The results derived can serve as a guide for the improvement plan. In addition to equipment and facilities of the building for passive disaster prevention, it needs to make a careful assessment of internal management aspects of disaster prevention, accommodated persons, nurse-to-patient ratio, and so on.