In extracting average velocity fields, the choice of different datum planes can lead to different timetodepth conversion results. In the twostep static correction used at present, there are two datum planes to construct the average velocity field and perform the timetodepth conversion: one is the velocity spectrum analyzing datum plane, i.e., the CMP datum plane, and the other is the final seismic datum plane, i.e., the common datum plane. The velocity field and timetodepth conversion were studied on these two different datum planes, and a method was put forward for deriving a smoothed nearsurface plane for timetodepth conversion by averaging all elevations of each receiver and each shot point within the scope of a CMP gathered. A case study shows that this method can provide higher precision velocity field and structure map without using any borehole data than the routine velocity field study method on common datum plane.