Abstract: Arc action in plasma torch with large anode nozzle and long arc passage has been researched by means of electromagnetism and hot fluid mechanics.It has been showed that the special phenomenon will emerg when temperature gradient and electric field gradient act on fluid.The results indicate that high temperature effect of arc column strongly prevents cold gas current from getting in,so as to restrain CVD process.Newly emerging colloidal particles deposit progressively on passage wall under the action of heatphoresis.Above mentioned problems can be solved effectively by changing entrance positions of different gases,reducing gradients of temperature and electric field as well as adding radial circular magnetic field.