The lower layer of magnesium crystal plate reinforced concrete composite floor is a prefabricated new fire-resistant magnesium crystal plate, and the upper layer is cast-in-situ reinforced concrete plate. In order to study the fire resistance and fire resistance rating of magnesium crystal plate reinforced concrete composite slab under different loading ratios, three composite slab specimens were tested subjected to ISO-834 standard temperature rise, and the fire time-temperature curve and fire time mid span-deflection curve at different positions of the composite slab were obtained. The test results show that the composite slab has good fire resistance up to more than 1.5 hours. The fire resistance of the specimens was analyzed by using the finite element software ABAQUS, and the reliability of the established model was verified by comparing with test results. The effects of loading ratio, thickness of magnesium crystal plate, thickness of concrete protective layer, strength of reinforcement and concrete on the fire resistance limit were investigated via the established model. The analysis shows that the load ratio and the thickness of magnesium crystal plate have great influence on the fire resistance of the composite slab. Based on parametric analysis, the fire resistance of composite slab is proposed for different loading ratios and thicknesses of magnesium crystal plate.