Abstract: On the basis of structure and principle of the fiveaxes linkage NC welding machine introduced in abstract, it mainly studied the movement control algorithm of the automatic welding for the complex intersection seam.Take the ellipsoidalwelding conjugation tube as the example, it separately introduceal the constructionof space curve equation of the intersection seam, the discrete point parameters while interpolating and fitting the intersection seam using line interpolation, the adjusting calculation of welding torch and the method of how to realize the servo control system with digital I O board and step motor.To control automatic welding process of complex intersection seam using the software algorithm introduced, the system becomes more versatile and the cost of equipment can be decreased considerably.