The invention relates to a method for improving prestack migration imaging accuracy by mean square root speed in the fields of seismic data routine processing, depth migration processing and seismic explanation, dynamically correcting and stacking common midpoint gather data by a series of preset normal speeds in the time and space domains, respectively, converting the prestack common midpoint gather data into common midpoint stacked speed gather data, separating the common midpoint stacked speed gather data into common normal speed section data, and making filtration and 2D weighting processing on seismic amplitude values in the normal speed scanning gather at each common midpoint to obtain a vertical mean square root speed spectrum. It is suitable for any complex construction condition. The obtained results are reliable and the speed resolution reflected by the mean square root is higher.