In view of the limitation in temperature field measurement of welding arc by two-dimensional arc spectral method, the detection of light radiation intensity at arbitrary points within arc based on three-dimensionally spectral acquisition system with confocal optic path as the key is studied. With arc light radiation intensity at each point on the central line (this line was on an arc transverse) perpendicular to detecting direction, Abel Inverse Transform is used for a reference for anti-interference de-coupling, in order relative emission coefficients of light radiation intensity at arbitrary points within arc to be rebuilt. Interference problems resulted from focally spectral measurement at an arbitrary point within arc using the studied three-dimensionally spectral acquisition systemwere solved. According to rebuilt relative emission coefficient about Ar Ⅱ characteristic spectrum (wave length arc 771.308 and 856.221 nm) of arc in micro-plasma arc welding with 2 mm torch height and 2 A welding current, three-dimensional temperature distribution of arc was obtained by relatively spectral intensity. Furthermore, Arc temperature distribution and arc geometry achieved in this way were discussed, and temperature field was compared with the numerical calculation results under same welding conditions. It is shown in the study that the studied three-dimensionally spectral acquisition system can be effectively used to acquire light radiation at a point in arc three-dimensional space. Radially spectral maps were non-axisymmetric with elongation due to a limitation of confocal optic path. However, the radially spectral maps with anti-interference decoupling were axisymmetric. Although the distribution of arc light relative emission coefficients with interference resistance de-coupling appears off-axis phenomenon, light radiation at the off-axis arc center reached maximum. The maximum light radiation at the location from nozzle to workpiece reduced firstly and then increased. This distribution is in agreement with “two-peak” distribution of arc light radiation. Moreover, when the location of arc was from nozzle to workpiece, arc radius decreased firstly, then kept certain value followed by increasing, leading to a quasi-column profile. This arc profile agreed with short arc geometry (height of arc torch was 2 mm) relative to welding current 2 A. In addition, the maximum value indirectly detected for welding current 2 A using three-dimensionally spectral acquisition and anti-interference de-coupling is in the temperature range of arc in micro-plasma arc welding. Furthermore, radial temperature distribution detected in this study is in agreement with the results by numerical simulation for temperature field. The maximum error was only about 0.03.