Abstract: First of all, the charactristics of commutating process in secondary invertion of variable polarity square waveform power source are analyzed in this paper, the switched voltage is lower and the current is higher then those are in first invertion.The flywheel effect of the welding loop plays an importent rule in the commutating process, it can provide suitable time for reverse driving switch elements,or promote arc re striking induced voltage.The objectve case is shown that the reverse driving delay time t_D isn't equal to arc current interrupted time t_D (i.e.arc extinguishing time).Tow kinds of the polarity exchanged methodes, i.e.t_d>0 and t_d<0, are induced, the mechanisms on their commutating processes are discussed fmther, and corresponding control strategies are proposed,then the realized methods and the experimental results are given Finally,distinguishing features of the two kinds of control methods and their suitable current limits are noted.From this,the commutating theory of square waveform power source is developed and it makes further foundation for improving operation performance and reliability of the source and for its reasonable design.