The optical properties and microstructures of YbF3 thin films by thermal evaporation and argon ion-assisted deposition are studied. The transmittance,refractive index and extinctive coefficient of YbF3 thin films are measured by photometer and ellipsometer,respectively the cross-section and crystalline phase structure are characterized by scanning electron mircoscope (SEM) and X-ray diffraction (XRD) meter. The experimental results show that the structures of YbF3 thin films deposited by the two methods are all amorphous;YbF3 thin films deposited by ion-assisted have a higher packing density,the packing density of YbF3 thin films increases and the peak of water absorption decreases more significantly than that by resistance-heating evaporation,and the adhesion is better. Moreover,the higher-energy argon ion-assisted deposition under 120 V bias voltage can get YbF3 thin films with higher packing density close to 1 and almost completely eliminating the loss of water absorption.