Abstract: In underwater acoustic engineering, improving precision of acoustic measurement for materials in free-field is very important in design of acoustic stealthy of underwater vehicle. In this paper, a numerical method to evaluate the edge diffraction effect on sound insulation measurement in free-field was investigated using finite element method (FEM) and boundary element method (BEM) based on the commercial sound and vibration analysis software LMS-SYSNOISE. The theoretical analysis model, analysis condition and analysis procedure are on the basis of real measurement. Numerical calculation and analysis were carried out for the influence of edge diffraction on both axial and off-axial receives in a underwater normal sound insulation measurement of a steel plate. The results show that edge diffraction effect is strongly dependent on the ratio of plate dimension to measured frequency wavelength, and the geometrical configuration of sound sources, plate and receiving field point. In addition, compared with axial receiving, off-axial receiving with small angle is in favor of restraining edge diffraction effect with certain frequency ranges, and the restraining efficiency is associated with off-axial distance and relative axial distance.