To prepare high-quality helical fiber devices， a heating method for an arc fiber with four electrodes was developed. A finite-element simulation model of arc plasma discharge with four electrodes was established based on the magnetohydrodynamics model. The effects of the electrode angle and distance and applied voltage on the temperature field of arc discharge were studied. A temperature-field simulation diagram was created based on the actual situation， and it indicated that the electrode rod with four electrodes formed a wide constant-temperature region. The wide constant-temperature zone was conducive to the release of fiber stress and reduced the influence of fiber migration. Based on the simulation structure， a plasma hot melt torsional-processing system with a wide constant-temperature zone for arc discharge with four electrodes was developed. When the maximum temperature of the heated fiber in the system was approximately 1 050 ℃， the axial constant-temperature zone length of the heated fiber was approximately 2.12 mm. Finally， helical long-period fiber gratings with different optical fibers and different periods were prepared by using the developed system. The experimental results revealed that when the wavelength of the prepared helical long-period fiber gratings was 1.21~1.3 µm， the intensity fluctuation of the transmission spectrum was less than 1 dB and the average intensity was higher than -1 dB. At wavelengths of 1.3~1.35 µm， the average light intensity was higher than -1 dB. The deepest trough of the transmission spectrum was >22 dB. To further verify the performance of the system， helical fiber structures with different periods were prepared using a single-mode， eccentric core optical， and eccentric twin-core fibers. The results showed that the cladding boundary of the fabricated helical fiber structure was clear and straight and no significant thread structure was present. No distinctive helical machining marks were observed in the central core of the fiber. The off-core of the fiber was smooth and continuous.