To improve the stability of magnetorheological jet polishing (MJP), a processing method of flow-focusing-assisted MJP is proposed in this paper. A relevant experimental setup was developed based on this method. The effects of the parameters (gas pressure drop, diameter of the small hole, and focusing distance) in the flow-focusing technique on the jet stability were studied by computational fluid dynamics simulation and experiment. The results show that the optimal gas pressure range is from 0.6 to 0.7 times the jet pressure, the optimal diameter of the small hole range is from 2.1 to 2.3 times the nozzle diameter, and the optimal focusing distance range is from 3.5 to 4.5 times the nozzle diameter. This is a preliminary attempt to provide a reference for future experimental studies of flow-focusing-assisted MJP.