The paper is devoted to investigation of spectral parameters of constricted arc plasma, as a foundation for further development of basic approaches to intellectual controlling of plasma-arc technologies of material treatment and welding. The relevance of application of spectrometric analysis of the welding arc consists in obtaining the possibility of sufficiently accurate control of its temperature and energy input into the product being welded, recording initiation of weld defects (including internal pores), determination of atmospheric oxygen ingress into the weld pool, etc. The spectral composition, temperature and concentration of the constricted welding arc plasma were determined in the paper to more precisely define its physical characteristics at 80 and 100 A currents, in order to establish the further applicability of dynamic spectral analysis of a constricted plasma arc in control of welding processes. As modern spectrometers with CCD-detectors of the spectral range of 200÷1100 nm have the resolution of the order of 0.35 nm, which 2-3 times exceeds the quantization step by the measured wavelength, and may lead to considerable errors, an approach is proposed to increase the accuracy of spectrometric measurements to acceptable values. Radiation spectra of the constricted welding arc measured in the wavelength range of 650÷1000 nm allowed within the model of local thermodynamic equilibrium calculating the excitation temperature of the main spectral range components: Ar I and O I. It was established that the excitation temperature of electron levels of Ar atoms in the welding arc plasma only weakly depends on the discharge current, and it is equal to 13500 ± 500 K. Excitation temperature of O I electron levels was equal to 10000 ± 500 К at arc current of 100 A and to 7200 ± 500 К at 80 A current. Electron density of high-current welding arc plasma was measured by Stark broadening of Ar I lines (696.543; 772.4; 912.3 nm). It was found that with the rise of current of the constricted welding arc by 20%, an increase of charge concentration in the arc plasma by 1.5 times and of its temperature by 1.2 times are observed.