The defect in multilayer of mask blank can degrade the image quality in EUV lithography. It is caused by particles deposited under or inside the multilayer, which leads to deformation of the multilayer structure. This deformation can impact the reflectivity of the multilayer and cause amplitude changes and phase shifts, resulting in pattern shift or critical dimension error in EUV lithography imaging. In order to improve imaging performance, medium size defects can be mitigated by pattern shifting techniques, while small size multilayer defects can only be compensated by mask repair to meet the imaging quality requirements. Therefore, it is necessary to investigate the effect of defect size on lithography imaging performance. In this work, we evaluate the impact of conformal defect with different sizes on the imaging of three classical test patterns: Line Space, Tip to Tip, and Tip to Line. By representing the defect by using ℎtop, 𝑤top, ℎbot, and 𝑤bot, the defects on different central and edge positions in the arc slit of exposure field are investigated. Then, the data gathered from lithography simulation is used to model the parameters of defect and the imaging result indicators such as CD. The corresponding relationship between the defect size and the imaging CD is derived through a mathematical model. From the constrains of the lithography imaging performance indicators, the tolerance of the parameter of the multilayer defect is further predicted inversely.