Abstract: From domestic research on soft-switch converse power source, it is always used in communication and aviation power supply, Although the application of the soft switch technology in the communication and aviation power supply is almost successful, its application in the welding power supply is just beginning because of the specialties of the electronic arc load. Therefore, the LCL type resonant structure is to be studied in this paper. Now,all welding power source almost adopt series-resonance structure,it has narrow soft-switch range and big duty cycle loss. On the base of the above actuality, this paper presents the principle of LCL structure and it's typical waveforms, meantime, presents the operation of LCL-type resonant arc welding power supply with fixed frequency and it's steady-state mathematical model. Using MathCAD to get the results and compared it to the results of the SPICE simulative experiment, we verify this mathematical model is correct.