Abstract: As the strong coherence of the sound wave in nearfield of a projector , the pressure in the region is very complex. In addition, hydrophones in practical applications usually have a certain size of their receptive face, and maximum dimensions of the so called "big" receiver may be many times the size of sound wave length. The pressure received by them is assembly pressure in loeal field, and will produce serious effect on results measured . In this paper, a numerical simulation method is used to study the radiation pressure field of a piston projector, smoothing effect on the field by the "big" receiver and their influence on measurement of sound absorption coefficient in nearfield, and the nearfield effect setoff method (NESM) is advanced. The calculated results show that this method not only widens the measuring range in nearfield, but gives more accurate results.