The invention relates to a power transmission line insulation apparatus. The apparatus comprises a housing, an insulation inner core and an outer seal. The housing comprises a matrix part and an extension part. The matrix part is a cylinder with a side opening, the center of the cylinder is the inner cavity of the matrix part, the inner cavity of the matrix part is used for accommodating oppositely connected power transmission lines, the extension part comprises an upper triangular extension sheet and a lower triangular extension sheet, the recessed cavity in the extension sheets and the inner cavity of the matrix part constitute an insulation inner core accommodation cavity together, the insulation inner core is placed in the insulation inner core accommodation cavity, and the outer seal is used for sealing a slit in the housing. The apparatus provided by the invention is applied to vertically crossed power transmission lines and has the advantages of rapid installation, good insulation performance and the like.