In the engineering design of ultra-high voltage isolated transmission lines, due to the concentrated load of the tension string and the interphase spacers, it is difficult to calculate the length of the interphase spacers accurately, which leads to difficulties in construction and installation, corona loss, and abrasion of wires. For this reason, the accurate calculation method for calculating the length of interphase spacers in ultra-high pressure isolated gears considering the influence of concentrated loads is proposed. By fully considering the influence of the weight, quantity, installation position and other factors of the concentrated load on the calculation of the length of interphase spacers, the calculation accuracy is improved. Firstly, the conductor sag formula considering the influence of the concentrated load of the tension string and the interphase spacer rod are introduced. Secondly, the calculation method for the length of the interphase spacer rod when the tension tower head is the same, different and has the number of rotation angles is proposed. Finally, the accuracy of the method in this paper is verified via engineering examples and measured values. The results show that the traditional method ignoring the influence of the concentrated load of the interphase spacer bar leads to a large error. Comparatively, the method proposed in this paper is more accurate than the traditional method and overcomes the above short-comings. The research results can provide references for the theoretical research and engineering design of the length of the interphase spacer.