For the 120 keV positive ion source, the insulation support system for the accelerator was designed, and the connection mode and basic parameters of the insulators and support flanges were determined. The optimization design of the insulation support system was studied through the finite element analysis method for the problems of electric field concentration and connection support. The electrostatic simulations of insulators and grid plates were carried out to determine the material and structural parameters of insulators, so as to study the insulating properties of the accelerator. The study shows that the maximum electric field around each insulator is less than 4 kV/mm, and the maximum electric field between grids is about 14 kV/mm, which can meet the voltage withstand requirements of 120 keV positive ion source accelerator. Secondly, considering the vertical installation of ion source, the connecting bolts of EG support flange and insulator would bear great normal stress and shear stress under the action of ion source gravity, thus the mechanical properties of the accelerator were studied. After mechanical analysis, the maximum normal stress of the bolt is 26.336 MPa, and the shear force is 1.292 MPa. The maximum normal stress of the bolt in the finite element analysis is 25.867 MPa, which is 1.78% different from the theoretical solution and less than the tensile strength of the material. The maximum shear force is 1.295 MPa, which is 0.23% different from the theoretical solution and less than the shear strength of the material. The results show that the mechanical properties of the 120 keV positive ion source accelerator meet the design requirements.