This investigation presents theoretical predictions and experimental results of the effect of a magnetic field intensity, initial arc length, and arc current on the deflection of a welding arc in a magnetic field. The sensitivity characteristics of an arc sensor using magnetic arc oscillation were also analyzed for gas tungsten arc welding. First, a mathematical model of the magnetic arc deflection for theoretical prediction was introduced, then the model was confirmed by corresponding experiments. Finally, a model of an arc sensor using magnetic arc oscillation was set up mathematically by considering the model of magnetic arc deflection and electromagnet, as well as welding arc and welding power source. Experiments and simulations were carried out to investigate how welding conditions such as initial arc length and arc current affect the sensitivity of the arc sensor. The simulated results based on the model showed good agreement with experimental ones.