This work aims at improving the measurement accuracy of arc length using the spectrum of arc light at a particular wavelength during gas tungsten arc welding with argon shield. To this end, effects of welding parameters on spectral distributions were studied. To verify the effects of base metal and arc length, the arc column was also sampled horizontally as layers for spectral analysis. Results showed that spectral lines of argon atoms are determined by arc length and independent of welding parameters other than the arc length. Based on these findings, a compact arc light sensor has been designed to measure the arc length with an adequate accuracy. A closed-loop arc length control system has been developed with the proposed sensor.