The interface slip of simply-supported steel plate-concrete composite slabs under bending moment was analyzed by using the reduced stiffness method, the function of the interface shear with shear span was established, and then the calculation formula of deflection under single-point loading and two-point loading was deduced. Meanwhile，assuming the steel plate-concrete composite slab as a type of reinforced concrete with the steel bars anchored at the bottom，based on the stiffness formula of the reinforced concrete beams, the design method of flexural stiffness of composite slabs. was obtained and the mid-span deflection of the composite slab was derived by the material mechanic formula. The slip effect between the steel plate and concrete was considered by using the method of the modified width reduction of steel plate. These two methods were compared with the experimental results of 9 simply-supported composite slabs, which were in good agreement. Both the two methods can be used to calculate the deflection of composite slabs, the second method has some safety reservation, and the second method is convenient to the design and application of the practice engineering.