The invention relates to a three-dimensional method for determining the structure of an underground oil-gas reservoir. The method comprises the following concrete steps: obtaining a velocity field and a density field of transverse waves and longitudinal waves conforming to underground geologic structure by processing; carrying out linear encryption interpolation on a dispersion field renewedly according to observation system parameters and the dispersion field so as to adapt the requirement for rebuilding a wave field; rebuilding wave field values on each net point at each moment on the basisof the dispersion field; outputting a pure P wave field and a pure S wave field at a position where a demodulator probe is located at each moment to be used as a wave field rebuilding record of the moment; and comparing stacking and excursion sections obtained from wave field rebuilding and stacking and excursion sections obtained by processing field collected data to determine the configuration and the position of the structure of the underground oil-gas reservoir. In the invention, the recoded longitudinal wave and the recorded transverse wave obtained from wave field rebuilding are completely separated so that the separation of the longitudinal wave field and the transverse wave field is not needed in the subsequent wave field research.