Abstract: Based on wave equation a fundamentl formula for plane wave propagation is derived, the theoretical relationship of inverting wave velocity structure through the observational wave field is discussed in this paper. The observational wave field can be decomposed into the surface plane wave field by using -p transform. The maximum amplitude curve in the plane wave field can show stably the change tendency of wave velocity with depth in the Earth. This property can be used to restrict the solution space. By using the wave field continuation method more useful information from the observational wave field can be extracted and the inversion solution not only can be obtained simply and quickly, but also is stable and less influenced by the subjective factor. The wave field continuation is a fine inversion method.Theoretical analysis and numerical modelling are carried out in the study of wave field continuation. By applying homomorphic decovolution the signal-tonoise ratio is improved.Finaly a sonar refraction profile in the northern part of the South China Sea is interpreted and computed. It is found as a result that there is a velocity interface from 1.76 km/s to 2.21 km/s at the depth of 1.4km. The velocity gradients in the upper and lower layer are 0.54 kms-1/km and 0.63 kms-1/km respectively. A discussion of the characteristics of shallow sea structure in the view of tectonic movements is given.