Based on the creative design of nozzle structure, the application fields of the micro-particles production using supercritical fluids (SCF) can be further broaden. According to the analysis of the structural characteristics of the nozzles, a new kind of nozzle with a tunable annular gap is proposed in this research, which includes a cover, movable trays, unmovable trays, seal fittings and connecting components. By virtue of FLUENT software, the flow field of the nozzle in outer mixing mode with two passages is numerically simulated. The effects of the passing length of SCF before mixing and the height of conic convexity on the flow-field are analyzed. The simulation results show that the above length does adverse effect on enhancing mixing extent, however, the smaller height of convexity can help achieve better mixing level. Then the nozzle is redesigned in which a movable tray and an unmovable tray are removed. The improved nozzle has simple and reasonable structure and can do better work on improving gas-liquid mixing extent.