The utility model provides a built -in CT assembly structure, include the level setting board I and board II, fasten in the insulating paper tube of interior phenolic aldehyde and the insulating paper tube of outer phenolic aldehyde of built -in CT between II of board I and board, vertical setting and fasten in between the two stay, set up in II upsides of board and connect in going up of interior phenolic aldehyde insulating paper tube upper end press from both sides wood and set up in I downside of board and connect in the lower clamp of the insulating paper tube lower extreme of interior phenolic aldehyde wooden, the insulating paper tube both ends of interior phenolic aldehyde stretch out board II and board I respectively and press from both sides wood and press from both sides wooden level down fixed through last, insulating paper tube of outer phenolic aldehyde and stay both ends are not all stretched out board II and board I and are pressed from both sides wood and press from both sides down that wood is vertical to be fixed through last. The utility model provides a built -in CT assembly structure, the use of saving metal material to avoiding the metalwork to leave the electric field, cross closely or imperfect earth etc. Easily causes and discharges, to the unfavorable problem of safety operation of transformer. Reduce the material kind, simple structure is reasonable, presss from both sides tightly firmly, saves the cost, has better insulating effect.