The invention discloses a method for predicting sound insulation properties of a flat plate based on an optimal square approximation theory. First, uncertainty is quantified as an interval model according to uncertainty test data; second, an optimal square approximation order of an interval parameter and a Gaussian integral point are determined based on the nonlinearity degree of inherent attributes of a flat plate sound insulation system with respect to the interval parameter, and an optimal sound insulation property frequency band is defined by utilization of an inherent attribute value of the flat plate sound insulation system at the position of a sample point and the optimal square approximation theory; finally, an optimal square approximation order of the flat plate sound insulation amount with respect to the interval parameter is determined, and a Gaussian integral point is utilized to sample the interval parameter; at the position of any frequent point within the optimal sound insulation property frequency band, the sound insulation amount interval limit is calculated through the sound insulation amount of the flat plate at the position of the sample point and the optimal square approximation theory, and the sound insulation properties of the flat plate under the influence of uncertain factors are output. According to the method, the influence of uncertainty on the inherent attributes of the flat plate sound insulation system and the sound insulation properties is considered, and the method can be used for guiding engineering application of the uncertainty.