Extreme ultraviolet (EUV) lithography system uses a reflective mask structure with multilayers of Mo/Si, but defects can easily occur in the surface of substrate or in the process of depositing multilayer. Even a small defect can cause a large disturbance to the reflected field of mask. Manufacture of defect-free EUV mask is one of the most critical challenges for EUV lithography. In this paper, a multilayer structure model of ultra-ultraviolet mask with defects was established, and the influence of defect size and location on the reflection field distribution of mask multilayer structure was analyzed by finite difference time domain (FDTD) method. The results show that the interference degree of the reflection field of the multilayer structure is the result of the combination of the height and width of the defects, and is related to the smoothness of the defect structure. The interference degree of the reflection field is also related to the height of the defects in the multilayer structure. The defects that cause the multilayer structure to be deformed near the bottom have little influence on the reflection field, while the defects that cause the multilayer structure to be deformed near the top have obvious interference on the reflection field.