Abstract: In order to prepare the Mo/B4C multilayer mirrors in 7 nm, the stress and thermal stability mechanism in Mo/B4C multilayers with different annealing ways was studied. Firstly, the periodic Mo/B4C multilayers with 60 bilayers were deposited in the fused silica substrate and Si substrate by DC magnetron sputtering method, the design periodic thickness(D) is 3.58 nm, the ratio of the thickness of the Mo with D is 0.4. Secondly, the prepared samples were annealed with various annealing ways, the maximum annealing temperature was 500 ℃. Finally, the period, the interface roughness and stress were measured using X-ray grazing incidence reflection, X-ray scattering and optical interferometer before and after annealing, respectively. The test results show that the vacuum annealing method can effectively reduce the stress of Mo/B4C multilayers, and the period and interface roughness of the Mo/B4C multilayers have no significant changes, which prove the Mo/B4C multilayers have good thermal stability within 500 ℃.