Behavior of welding arc plasma,characteristic of welding voltage,local changes of electron temperature and electron density were investigated respectively with high speed camera,inductance gauge of current and voltage,spectrometer in low power YAG laser-gas metal arc welding (GMAW) hybrid welding process comparing with simple GMAW welding case. Based on the experimental results,behaviors of hybrid welding arc plasma were explained by plasma theory of thermodynamic equilibrium. The results indicated that,due to the input of YAG laser,the GMAW arc was attracted and constricted,electron temperature and electron density of local arc were increased to (15400±900) K and (1.265±0.101)×1017cm-3,the voltage output of GMAW source was decreased,and the formation of local thermodynamic equilibrium (LTE) of hybrid welding arc plasma was accelerated.