In this paper, first-principles molecular dynamics simulations are applied to study the ionic structure and electronic properties of KF-NaF-AlF3 system. The results show that the main forms of complex ion groups are [AlF4](-), [AlF5](2-) and [AlF6](3-). With KF concentration increasing, the covalent interaction between Al-F ions first increases and then decreases, resulting in the increase of Al-F coordination number and then decreases. The calculation results of Raman spectra on [AlF4](-), [AlF5](2-) and [AlF6](3-) are close to the experimental values, which indicates FPMD is applicable to similar molten salt systems.