In the engineering design of ultra-high voltage isolated transmission lines, due to the influence of 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, a method is proposed for calculating the length of interphase spacers on ultra-high voltage isolated tower considering the influence of concentrated loads. 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 formulae are introduced considering the influence of the concentrated load of the tension string and the interphase spacer. Secondly, a length calculation method is proposed for the interphase spacer when the tension tower head have the same size, or have different sizes, or have certain rotation angles. Finally, the accuracy of the method in this paper is verified via engineering examples and measured values. The results show that by considering the influence of the concentrated load of interphase spacer bar, the method proposed in this paper has a smaller calculation error and is more accurate than the traditional method. The research results can provide references for the theoretical research and engineering design of the length of the interphase spacer in isolated gears.