Abstract: Several aspects of the dynamic characteristics of ZX5-400 silicon controlled are welding power supply are analyzed in detail in this paper. The results show that because of the sharp current rise during the short time transfer from load to short-circuit condition, the breakdown of the small metal bridge formed by over 65% droplets in the short circuit transition takes place in the decreasing section of current and even also in the stable section of short-circuit current when low-hydrogen electrodes are used in welding. This is very beneficial to diminishing spatter. Meanwhile, a new means for examining the dynamic characteristics of this kind of welding machine is put forward. The parameter i_fd/I_f(t_o) is proved to be good for showing precisely the relation between dynamic characteristics of power supply and spatter.