In order to study the effect of base pressure on the microstructure and optical properties of Yb/Al multilayers, a series of SiC/(Yb/Al)₃ period multilayers with the same structure were prepared by DC magnetron sputtering under base pressure conditions of 4×10^-5 Pa, 8×10^-5 Pa, 1×10^-4 Pa, 2×10^-4 Pa and 4×10^-4 Pa, respectively. The surface and internal structures of Yb/Al multilayers were characterized by X-ray grazing incident reflection, atomic force microscopy and wide-angle X-ray diffraction. The results show that the average interface width of Yb/Al multilayers decreases from 2.15 nm to 1.82 nm with the raise of base pressure from 4×10^-4 Pa to 4×10^-5 Pa; the surface roughness decreases from 1.87 nm to 1.43 nm. Yb, Yb₂O₃and Al polycrystalline grains are formed in the film, and the grain size increases slightly. The stress of SiC/(Yb/Al)₃ period multilayer is tensile, and the stress increases from 85 MPa to 142 MPa with the base pressure raising from 4×10^-4 Pa to 4×10^-5 Pa. The reflectivity of the sample prepared under 4×10^-5 Pa base pressure was measured. When the wavelength is 73.6 nm and the incident angle is 5°, the reflectivity of the sample is 31.3%.