The occurrence of earthquake disasters poses a great threat to the safety of human life and property. To ensure the safety of buildings during earthquakes, determination of the load, initial stiffness, ductility coefficient, and vertical displacement of buildings is required. If the gravity load is higher than the peak value, the building safety cannot be guaranteed. To analyze the gravity load capacity of the top floor of a building during a horizontal earthquake, we established a three-story, three-span building model to test the lateral stiffness of the building under gravity load. Then, we studied the influence of the gravity load on the detected stiffness of the top structure of the building during a horizontal earthquake. We selected five specimens from the model, and analyzed their material properties, experimental results, and load displacements. The peak load of the building, as determined by the gravity load displacement curve of the experimental model, was 700 kN. That is, when the peak load of the top floor of the building exceeds 700 kN, the safety of the building cannot be guaranteed.