The characteristics of flow field and the brine distribution will greatly affect the cavern expansion and final cavern shape in the process of constructing salt cavern gas storage, while the flow characteristics and concentration distribution can't be monitored by technical means. To study flow characteristics, brine concentration distribution and their impact on cavern shape expansion during the multi-step salt cavern construction, numerical simulation calculations were conducted based on the established multi-physics coupled flow model, and the results were compared with field data of two storage wells in Jintan. The simulation results were consistent with the field data, which verified the accuracy of the model. Based on the simulation results and data, the flow field characteristics and brine concentration distribution in each stage of solution mining were analyzed, and how flow characteristics and brine concentration distribution affect the cavern expansion was analyzed. The results show that there will form a vortex in cavern during positive circulation, which mainly cause erosion and inhibits dissolution during solution mining. There will form two vortices in cavern during reverse circulation. The one in lower part promoting the discharge of brine and the another one in upper part promoting dissolution Forced convection will cause vortex, and the strength of vortex will gradually decrease with the expansion of the cavern. The vortex makes the fluid flow in the cavern, and promotes the process of convection diffusion, thus affecting the distribution of brine concentration, and ultimately affecting the expansion of the cavern. The shape expansion of the cavern can be controlled by adjusting the positive and negative circulation mode, the position of the tube, the position of the oil cushion, and the injection rate. In the early stages of cavern construction, erosion plays a dominant role, which resulting in high cavern construction efficiency, resulting in a slower cavity making rate because of the small surface area of the cavern. The cavern mainly extends towards the middle and lower