Abstract: Tank is one of the most important parts of tandem accelerator, working as the paltform to carry other components in the main body of accelerator. Previously, most mechanical designs for tandem accelerator tank were based on experience or reference to similar foreign products, without a sound theoretical basis. Regarding this, theoretical design research on 2×6MV tandem accelerator is conducted in this article. The purpose of mechanical design for the 2×6MV tandem accelerator is to reduce the tank's mass by controlling the wall thickness while ensuring its structural strength and airtightness. According to the membrane theory, the tank has the standard elliptical head, and the minimum nominal thickness of tank's wall thickness is 9 mm; Based on the approximate analysis and calculation method proposed by L.P. Zick for horizontal vessels, distance from the center of the saddle base plate to the tangent line of the head is 0.64 m. The finite element model of tank that stand constraints and external loads is established to conduct stress and displacement analysis. Through the research, the shape of the head, thickness of the outer wall, and position of the saddle are designed and selected, and mechanical parameters that minimize the mass of tank are obtained. It provides a reference for manufacture of the 2×6MV tandem accelerator's tank and mechanical design of other tandem accelerators with the similar structures.