Abstract: Now it’s very difficult to select the discharge capacity for the low-pressure hydrocarbon reservoir of oil field in the sand-washing operating course of sanding accident well. The treatise applied hydraulics theory and site-data inversion technology to set up a calculating pattern of returned velocity of annular space in the course of sand-washing. we applied the pattern in SZ36-1-E21 well of Bohai Oilfield,and adopted the calculated result of the treatise in the sand-washing operation of the wall which made it very smooth, the speed of construction improved greatly, the effect of application is very good. Therefore, the result of research can provide foundation of theory for the selection of sited sand-washing operation, afford guaranty of technology for the safe construction of down-hole, reduce more the occurrences of complexity in the sand-washing operation,protect the oil and gas layer, and enhance operating efficiency.