Abstract: The principle of numerical method of finding the effective value of current in spot welding by using the controller of micro-computer,the controlling and measuring device,and the related accuracy are presented in this paper.This paper elaborates the solving process of peak current and draws an important conclusion that within the range of precision of the micro-computer the ratio of effective value of current I to peak current I_M bears no relation to the power factor of load φ and is only a function of the angle of current conduction θ of the silicon controlled rectifier.Thus,formula I/(I_M)=P(θ) is obtained.The result of this study offers new and viable method for the control and measurement of the effective value of welding current during spot welding.