Both U3+ and U4+ can stably exist in molten chlorides for fast-spectrum molten-salt reactors (MSRs), but the differences in their structure and chemical properties in the promising candidate salts molten NaCl and MgCl2, are not thoroughly understood. In this work we employ different molecular dynamics (MD) simulations to investigate the structures and transport properties of UCln (n=3, 4) in molten NaCl, MgCl2, and NgCl-MgCl2 mixture at various mole fractions. Our AIMD simulations reveal that the coordination number of Cl around U changes with the uranium concentration as the network structure of U-Cl-U is formed in both NaCl and MgCl2 salts. The different solvent salts lead to the distinct network formation in the mixture of NaCl-MgCl2-UCln (n=3,4). Long-time scale classical MD simulations also are performed to connect the network formation with transport properties and the figure of merits for heat transfer.