Abstract: The theory and related algorithm of the nonlinear parameter approximate entropy are introduced.An idea for analysing the arc stability in different transfer modes of CO₂ welding based on approximate entropy is put forward.With matching appropriate parameters and increasing wire feed speed, the transfer modes are found as follow :short circuit transfer, instantaneous short circuit transfer and buried arc transfer.Through calculating the value and standard deviation of approximate entropy of current in different transfer modes, the characters between approximate entropy and arc stability in CO₂ welding are studied.Approximate entropy of current signal can be taken as a criterion of arc stability in different transfer modes.The greater approximate entropy and less fluctuation indicate a more stable welding process.The approximate entropy theory as a criterion of arc stability is further perfected.