The microscopic structure and photoelectron spectra of an aqueous solution are investigated with ab initio molecular dynamics simulations and ambient pressure X-ray photoelectron spectroscopy (AP-XPS). The simulation results show that the structural fluctuations in an aqueous solution can lead to remarkable peak broadening (similar to 1 eV) of ionic species, which is in good agreement with the results from AP-XPS experiments. We find that this broadening of the XPS peaks can be directly correlated with the local structural fluctuations in the aqueous solution, such as the evolution of solvation shells. This work demonstrates that the rich dynamics of solvation shells in aqueous solutions can be revealed by combining advanced simulations with in situ AP-XPS, and may stimulate new developments in the in situ XPS characterization of complex electrochemical reactions.