Abstract: On the basis of plasma physics and optical spectroscopy theories,a new method named "Multilineal Spectroscopy Analytic Solution" for diagnosing welding arc plasma is proposed.By this method and the relevant measurement system,radial distribution of hydrogen concentration is measured,together with the corresponding temperature and pressure in the TIG welding arc.The details of the error analysis as well as the verification for the measurement results are presented.The technique described in this paper has many advantages:real-time,quantitative,and simultaneous measurement of multi-parameter space distribution.This technique can be applied in principle also to the measuring of other elements in welding arc,and may be considered as a new way to solve many practical and theoretical problems in welding engineering.