In the field of precision measurement and metrology, a vertical laser is a valuable measurement tool. Its applications include, but are not limited to, the measurement of vertical displacement and attitude in the Kibble balance and joule balance for kilogram realization. A vacuum compatible, vertical-laser alignment method based on an oil mirror and air-spaced doublets is proposed to measure and compensate the vertical deviation angle of the laser beams. Dimethyl silicone oil was selected as the natural direction reference, and the air-spaced doublets were designed as the focusing elements to make the deviation angle correspond to a distance of spots. The corresponding alignment system is vacuum compatible, nonmagnetic, and can be miniaturized. In addition to the mass traceability in the realization of a kilogram, this system can also be applied to the gravity measurements of outer space planets in the field of aerospace science. The off-axis error, which is the highly influential systematic error of the alignment system, is suppressed by replacing the plano-convex lens with a combined optical element-"air-spaced doublets+aperture." The performance of the alignment system has been investigated by experiments. The Type B uncertainty of the alignment system was evaluated to be 19.19 µrad.