Different concentrations of Er3+ doped in KPb2Br5 powders are prepared by solid-state reaction method. The upconversion spectra under the excitation of 980 nm laser are characterized and show the concentration-related change in emission band. With the doping concentration of 2.5 mol%, the sample exhibits mainly two green emission peaks at 530 nm and 550 nm, which correspond to the transitions of 2H11/2 and 4S3/2 levels to ground state 4I15/2 respectively. When the doping concentration of Er3+ is increased to 5 mol%, the upconversion emission is dominated by 490 nm, related to the transition of 4F7/2 to 4I15/2. Increasing the doping concentration up to 7.5 mol%, the emission quenching is observed throughout and the emission band appears mainly at 690 nm which comes from the transition of 4F9/2 to 4I15/2. The influence of possible substitution of Er3+ on lattice constant and upconversion emission properties of KPb2Br5 are discussed based on the first-principles calculation and Judd-Ofelt theory. The results show that the substitution of Er3+ in KPb2Br5 is dominated for Pb(1) in low concentration. When increasing the Er3+ doping concentration, it may substitute for Pb(2) Pb(1) and Pb(2) sites. The impact of the symmetry of the crystal field on the site-selective doping of Er3+ is proposed to explain the variation of upconversion luminescence spectrum.