The sound scattering from a finite flat plate is crucial in the scatter analysis in the water. Due to the reverberations and difficulty of determining the attitude of the finite flat plate in the water, the reflection or absorption coefficient is often used for the evaluation of the scattering properties, indirectly. A three-dimensional spatial method is proposed for directly measuring the sound scattering of a finite flat plate. The ultrasonic ranging method is utilized to determine the attitude of a finite flat plate in real-time. The complex cepstrum method is modified to extract the sound scattering from the reverberations. Using a three-dimensional spatial measurement method, a measurement system is established for measuring sound scattering from a finite stainless-steel flat plate, with a length of 1.6 m, a width of 1.2 m, and a thickness of 5 mm. The measurement results are consistent with the calculated results by a finite-element model.