A　method　that　alternating　sine　current　superposes　on　direct　current　was　adopted　for　study　on　alternating　current　impedance　of　the　small　current　TIG　arc.　It　was　found　that　with　the　frequency　getting　high,　the　modulus　of　arc　impedance　increased　and　then　decreased,　and　the　phase　angle　decreased　continuously.　The　maximum　of　the　modulus　just　coincided　with　the　zero　point　of　phase　angle,　and　obviously,　arc　impedance　had　a　resonance.　By　analyzing　the　voltage　and　current　data　in　many　different　frequencies,　the　electronic　model　and　transfer　function　of　TIG　arc　alternative　component　were　deduced.　The　parameters　of　transfer　function　were　fit　by　using　mathematics　software,　and　the　resonance　frequency　was　calculated.