The electroscope plays a vital role in the maintenance and operation of power lines. We propose a novel non-contact electroscope for high voltage direct current (HVDC) transmission lines using magnetic field detection to solve the safety hazards and risks of the contact electroscope. In this paper, the basic principles of the HVDC magnetic field electroscope are first reviewed. The magnetic field distribution around the ±800 kV HVDC power line under the operating conditions of single-phase full-voltage is simulated with finite element method. The alarm threshold value of the insulation tester is extracted based on its magnetic field distribution characteristics. The non-contact electroscope with magnetic field detection method has prospects in live detection of HVDC transmission lines.