The invention discloses a T-shaped cross-section column composite insulator device and an optimum cross-section size calculation method thereof, The device comprises an insulating core rod, a connecting fitting and an insulating jacket. The cross section of the insulating core rod is arranged in a T-shape. The shape of the T-shape cross section comprises a standard T-shape cross section and one ormore straight lines of the outer contour are arranged in a T-shape cross section of a circular arc with a preset curvature. The insulating mandrel is used for ensuring the mechanical performance of the insulator; The connecting fitting is used for connecting the post-type composite insulator and the power transmission and transformation equipment component, so that the post-type composite insulator and the power transmission and transformation equipment component are reliably connected; The insulating jacket comprises a plurality of umbrella skirts and sheaths, and the insulating jacket is used for protecting the insulating mandrel; The umbrella skirt is used for increasing the creepage distance of the insulator; The insulator device can be applied to overhead AC/DC transmission lines andsubstations with voltage grades above 1000 V in electric power, railways and power plants.