For evaluating the hygro-thermo-mechanical reliability of fan-out (FO) wafer-level package, a coupled-field finite element (FE) model was developed to evaluate the moisture concentration and stresses at materials interfaces in a FO package structure. A series of moisture absorption experiments were first carried out on epoxy molding compound (EMC) under various temperature and relative humidity (RH) conditions. Material parameters for moisture absorption models including saturation concentration, diffusivity and coefficient of hygroscopic swelling (CHS) were obtained from fitting to the experimental results. These models were then implemented in a coupled FE simulation to estimate the evolutions of moisture concentration, package warpage and peel stresses at EMC-Si and polyimide (PI)-Cu interface corners under moisture sensitivity level (MSL) testing. The peel stresses were considered as the driving forces of the interface delaminations caused by moisture absorption. The experimentally fitted moisture diffusion model and the coupled hygro-thermo-mechanical simulation model established in this study can be used for evaluate the effects of geometry and materials on the moisture sensitivity of FO wafer level packages.