Abstract: In order to study the similarity of temperature field and mechanical properties of concrete slabs with different proportions at high temperature, 12 groups of prototype reinforced concrete slabs and corresponding 70% and 50% scale model slabs were designed. The finite element model considering water migration was established. Based on similarity theory and heat transfer theory, the furnace temperature curve and gravity load of the scale-down model were modified, and the thermal-mechanical coupling analysis was carried out. The effects of load size, fire surface number, plate thickness, protective layer thickness, reinforcement method, boundary conditions and section size on the similarity of temperature field and mechanical properties of prototype plate and corresponding scale-down model plate at high temperature were studied. The results show that after modifying the furnace temperature curve of the scale model, the temperature fields of the scale-down model and the prototype have good similarity. When the gravity correction is not considered, the similarity between the deflection curves of the scale-down model and the prototype is poor in the case of double-sided fire, and the maximum relative error is 31.7%. After considering the gravity correction, the similarity of the deflection curve between the scale-down model and the prototype is greatly improved, and the average relative error is reduced to 2.3%. This paper can provide a theoretical reference for the fire scale test of concrete structures, and can effectively reduce the cost of structural fire resistance tests.