The migration of uranium (U) in the surficial environment has received considerable attention. Due to their high natural abundance and low solubility, autunite-group minerals play a key role in controlling the mobility of U. However, the formation mechanism for these minerals has yet to be understood. In this work, we took the uranyl arsenate dimer ([UO2(HAsO4)(H2AsO4)(H2O)]22-) as a model molecule and carried out a series of first-principles molecular dynamics (FPMD) simulations to explore the early stage of the formation of trögerite (UO2HAsO4·4H2O), a representative autunite-group mineral. By using the potential-of-mean-force (PMF) method and vertical energy gap method, the dissociation free energies and the acidity constants (pKa's) of the dimer were calculated. Our results show that the U in the dimer holds a 4-coordinate structure, which is consistent with the coordination environment observed in trögerite mineralogy, in contrast to the 5-coordinate U in the monomer. Furthermore, the dimerization is thermodynamically favorable in solution. The FPMD results also suggest that tetramerization and even polyreactions would occur at pH > 2, as observed experimentally. Additionally, it is found that trögerite and the dimer have very similar local structural parameters. These findings imply that the dimer could serve as an important link between the U-As complexes in solution and the autunite-type sheet of trögerite. Given the nearly identical physicochemical properties of arsenate and phosphate, our findings suggest that uranyl phosphate minerals with the autunite-type sheet may form in a similar manner. This study therefore fills a critical gap in atomic-scale knowledge of the formation of autunite-group minerals and provides a theoretical basis for regulating uranium mobilization in P/As-bearing tailing water.