Abstract: In order to assess the stability of CO₂ short circuit welding under waveform control, approximate entropy(ApEn) theory of nonlinear parameter was introduced in this paper.The characters of the current signal in the welding was analyzed by ApEn.The ApEn value was apparently different by the comparison of the current signals in different parameters, which could be thought as the criteria of welding process stability.The smaller the value and variance of approximate entropy is, the more stable the welding process is.The result shows that obvious effect can be seen on the complexity of characterization signal by ApEn analysis, which provides an effective way to analyze the stability of CO₂ short circuit transition under waveform control.