We used the first-principles molecular dynamics simulations combined with the interatomic potential molecular dynamics to study the ionic structure and transport properties of KF-NaF-AlF3 fused salt. Simulation results show that the ionic structure of KF-NaF-AlF3 fused salt is principally dominated by the distorted five-coordinated [AlF5]2- and six-coordinated [AlF6]3- groups. When melting to a liquid, a part of the six-coordinated [AlF6]3- group dissociated into the four-coordinated [AlF4]- and five-coordinated [AlF5]2- groups. Four, five and six-coordinated aluminum-fluoro complexes coexist in KF-NaF-AlF3 fused salt, while the tetrahedral [AlF4]- groups are relatively rare. The content of the bridging fluorine atom is relatively small, about 5-11%, which indicates that the polymerization degree of the ionic structure of the KF-NaF-AlF3 fused salt system is lower. The KF-NaF-AlF3 fused salt has better liquidity and ionic conductivity due to the high self-diffusion coefficients of all particles in the fused salt system. KF can effectively break the F atom bridges, which reduces the polymerization degree of the ionic structure of the fused salt system and increases its ionic conductivity.