Quartz crystal microbalance (QCM) vibrating in thickness shear mode is used to detect the mass changes of the layer deposited on its surface. In this paper, an AT-cut quartz crystal microbalance with partial mass absorption on its electrode surface is modeled with Mindlin's theory. The effect of position and percentage of covering area of the mass absorption is studied. The numerical simulation results show that QCM with larger electrode size is less sensitive to the percentage of covering area, and QCM with smaller electrode thickness is less sensitive to the position attachment of the mass absorption on its surface.