Traditional water-rock reaction experiments under high temperature high pressure (HTHP) conditions often result in precipitation or adsorption of the products during the subsequent cooling processes. This issue can be overcome by synthetic fluid inclusion, which enables in-situ entrapment of fluid samples under HTHP conditions, and analyses of fluid composition using a LA-ICP-MS. This study simulates the HTHP (200℃ and 10 MPa) water-rock reaction process between basin brine (NaCl/NaCl+CaCl₂) and basement rocks (Triassic diabase, lithic limestone, and Middle Triassic tuff) in Lanping Basin, Southwest China, and uses synthesis fluid inclusion in calcite to trap the fluid during reaction. By comparing the fluid composition before and after water rock interactions, we are able to explore the origin of ore-forming metals in the fluids, and their contributions to the formation of the Mississippi Valley-type (MVT) Zn-Pb deposits in the basin. Based on microthermometric analyses, the freezing temperature point of synthetic fluid inclusions trapping a 3 m NaCl + 0.15 m CaCl₂ fluid ranged between -13.6 and -11.4℃, while that trapping a 3 m NaCl fluid ranged between -11.8 and -10.7℃, indicating that the fluid composition trapped within these inclusions is consistent with the initial fluids loaded in our experiment. Furthermore, both microthermometric and LA-ICP-MS analyses demonstrated that this synthetic fluid inclusions method using calcite as the host mineral has a nice potential in simulating the relatively low-temperature fluid-rock interactions in nature.