Through the test of 6 unbonded prestressed two-way plates, the main factors affecting the deformation of two-way plates are studied, such as the structural form and the ratio of prestressed reinforcement. The influence of the side beam on the maximum deflection in the span of simply supported plate is converted into load and support condition coefficients. The effect of prestressed and non-prestressed tendons in two directions on the maximum deflection in the span of simply supported plates is reflected by the reduced ratio of reinforcement (ApQ) parameter, and the formula for calculating the short-term stiffness and deformation of unbonded prestressed simply supported plates is established. The calculated results agree well with the experimental results.