The invention relates to the field of oil and gas well cementation, in particular to a quantitative method for the well-cementation prepad fluid rational dosage. Through a displacement interface stability theoretical result and a prepad fluid flushing efficiency evaluation method, the influence rule of objective conditions such as the sleeve eccentricity degree, the borehole drift angle and the borehole diameter and subjective conditions such as fluid performance parameters and the displacement discharge capacity on the well-cementation interface stability and the mud cake scouring effect is comprehensively considered, and a calculating method for prepad fluid quantification is formed. The well-cementation quality is improved, and effective scouring of borehole wall and sleeve wall mud cakes, effective displacement of drilling fluid of all spaces of annuluses and effective isolation of cement slurry and the drilling fluid are achieved.