Abstract: The composition and temperature of aqueous vapor plasma arc are studied by means of the spectrum diagnosis. The diagnostic results show that the aqueous vapor plasma arc consists of hydrogen atoms, hydrogen ions, oxygen atoms, oxygen ions and free electrons mainly, the arc has high temperature, enthalpy and strong oxidation characteristics. The different medium aqueous vapor plasma arc temperatures are measured by spectrum. The different mediums are water, aqueous alcohol and aqueous acetone. The measure results show that water medium arc temperature is the highest, but the aqueous alcohol medium arc temperature is the lowest. And the arc temperature drops down with the consistency of aquesous alcohol or aqueous acetone increases. The cutting and welding characteristics of different medium aqueous vapor plasma arc are studied by experiments. The experiment results show that the aqueous alcohol or aqueous acetone, which the consistency of aqueous alcohol or aqueous acetone is 20-30%, has better cutting characteristic. But the weldig property of the aqueous vapor plasma arc is not satisfactory.