The nanoindentation in single crystal Ag thin film was simulated by a multiscale quasicontinuum (QC) method. The influences of the indenter width on the contact stress distribution, the load necessary for first dislocation emission and the nanohardness were investigated in the nanoindentation deformation process. In addition, all results of the simulation were analyzed with Rice-Thomson (R-T) dislocation model. Results show that the sharp drops of the load-displacement curves are due to the collective dislocation activities. The indenter width can greatly influence the contact stress distribution, the load necessary for first dislocation emission and the nanohardness. Normal contact stress, shear contact stress and nanohardness are decreased as the indenter width is increased, indicating an obvious indenter size effect in nanoindentation of single crystal Ag thin film; whereas the load necessary for first dislocation emission is increased as the indenter width is increased, and is in direct proportion to the square root of the indenter half-width. The simulation results are in good agreement with experimental results and R-T dislocation model