A selective flocculant (XN-1) for oil-based drilling fluids, synthesized from styrene, lauryl methacrylate and methacryloyloxyethyl trimethyl ammonium chloride, was developed to address the problem of the difficulty of removing the poor quality solid phase from oil-based drilling fluids. The flocculation mechanism was analyzed by sedimentation experiments and Zata potential tests, and the flocculation effect was evaluated by flocculation treatment of dumping fluid, old slurry and on-site oil-based drilling fluids in indoor and on-site. In the indoor experiment, when 2% NX-1 was added to the dumping fluid and old slurry, the low-density solid-phase content was reduced by 9% and 7%, respectively, and when 1% NX-1 was added to the on-site circulating drilling fluid, the final cut was reduced significantly; in the on-site test, when 2% and 1% NX-1 was added to the dumping fluid and old slurry, the low-density solid-phase content was reduced by 7.3% and 12.1%, respectively, and when 0.5% NX-1 was added to the circulating drilling fluid system, the growth of the low-density solid-phase content was effectively suppressed. inhibit the growth of low-density solid-phase content. Experiments show that XN-1 has good selective flocculation effect on poor solid phase, which meets the requirements of practical use.