This invention involves a method and a device for enhanced oil-water separation and desalination in a low-pressure separator. The water-containing oil is mixed with desalted water in a countercurrent way at the entrance, wherein the desalted water accounts for 0-1% of the water-containing oil by volume. The resultant oil-water mixture then enters a T-shaped liquid-gas separator (3) for degassing treatment to quickly separate gas from the mixture. In a low-pressure separator, the oil-water mixture flows, from left to right, to a flow conditioner (4) to uniformly distribute the mixture in the transverse section, and then flows to a hydrophilic droplet agglomeration module (5) and a CPI fast separation module (6) to separate water from oil, wherein part of the separated water is discharged and the oil with a trace of water (0-0.01%) passes over a partition (18) to a deep separation segment. The oil is subjected to deep water removal by a conjugated fiber water removal module and then discharged, and the water captured by the conjugated fiber water removal module is subject to a conjugated fiber oil removal module for deep oil removal and then discharged.