The space charge modified electric field is a key factor in the electromagnetic environment of DC lines. The accurate and efficient prediction and effective long-term test are of great significance for the stable operation of power grid. Therefore

a method was proposed based on an upstream finite element method whose initial charge density of wire was calculated from the unipolar flux line method. Meanwhile

continuous monitoring of the total electric field was carried out in the laboratory and under the actual DC transmission lines. The results show that the method proposed in this paper can be adopted to significantly improve the solution speed when ensuring the accuracy of the calculation. The test results in the laboratory are in good agreement with the calculation results. The minimum relative error is 0.41%

the maximum relative error is 8.3%

and the maximum absolute error is 0.722 kV/m. Although the continuous monitoring results of the space charge modified electric field of the selected two sections of ±500 kV Yihua line and ±800 kV Lingshao line were affected by environmental factors significantly

they were consistent with the calculation results and were in good agreement. The proposed method in this paper not only has a high solving rate but also ensures the accuracy of calculation. The measured results in this paper can provide data basis for the research of space charge modified electric field under DC transmission lines.