Abstract: In the Bohai Oilfield, small-sized double-layered inner pipes for one-trip multistage frac-packing string inside small-bore Ø177.8 mm casing cannot ensure the required large pump rate for frac-packing. In this case, perforations and frac-packing can only be performed stage by stage. This delivers a low completion time-effectiveness but high operation costs and failures to running production strings. After the application of conventional sand control packing tools in Ø177.8 mm casing was analyzed, a guideline for localizing a one-trip multistage frac-packing tool in Ø177.8 mm casing was put forward: increasing the wash pipe size. The I.D. of the sand control pipe string was increased from 76.2 mm to 98.55 mm, allowing for the running of a production string. The O.D. of the wash pipe in the service string was increased from 73.0 mm to 88.9 mm, delivering a sectional area up to 16.77 cm² in small annulus. This sectional area allows for a large open area and small friction. The reverse circulation pressure can be controlled within 20.69 MPa, allowing for one-trip multistage frac-packing. This tool was applied in 24 wells in the Bohai Oilfield in 2018 and delivered great economic benefits. At present, this tool is widely promoted in the Bohai Oilfield.