Cylindrical cyclones are a new technology for wastewater separation. The distribution characteristics of oil droplets significantly influences wastewater separation performance. In this study, the characteristics of oil droplet distributions in an oil-removing cylindrical cyclone were studied systematically, and the particle size changes in the cylindrical cyclone were determined under the conditions of a low oil content and different flow mixing speeds. The results show that the particle size distributions of inlet oil droplets have a LogNormal distribution. The distribution of the "inflection point"combined with the "extreme point"is easily obtained for the oil droplet particle size at the overflow mouth at a high inflow velocity. When the flow velocity is low, the particle size of oil droplets at the overflow mouth can exhibits a "double peak"distribution or a combined "inflection point"and "extreme point"distribution. For the oil-removing cylindrical cyclone, the oil droplet coalescence phenomenon exhibits the best performance when the split ratio is approximately 0.34 and the centrifugal acceleration is around 100 g, and the oil droplet coalescence effect and separation performance are also the best under these conditions. The results of this study can be used to design cylindrical cyclones in the field of sewage treatment. © Published under licence by IOP Publishing Ltd.