The voltage measurement and high-speed photography were conducted for a cascade plasma torch with an external magnetic field to study the voltage-current characteristics and key factors for suppressing the arc fluctuations in plasma torch. A ring-shaped permanent magnet was used to generate an external magnetic field of about 0.27 T at the center of the magnet. The operating current, the gas flow rate, and the anode-cathode distance were varied in the range 40-100 A, 10-40 L/min, and 0-10 mm, respectively. The width of a ring-shaped anode was set to 1 mm or 3 mm. The end-on images of the anode arc jets were captured by high-speed photography, and the associated arc voltage was measured. The time-averaged arc voltage of the cascade plasma torch shows a negative trend with the increase of the operating current with no external magnetic field applied. Higher time-averaged arc voltage is obtained by increasing the anode-cathode distance and applying the external magnetic field. The results of highspeed photography show that steep voltage drops are caused by successive arc root jumps on the anode. When an anode with 1 mm width is used, a periodic sine-like voltage waveform is observed at low gas flow rate. In that condition, an arc root rotates along the anode wall continuously with no observed breakdown process.