Abstract: An orbit-holding traveling mechanism of girth weld ultrasonic scanning equipment was designed in this paper. It held the orbit by engaged wheels with flange and drove the scanning equipment by a tooth-shaped driving wheel engaging with the orbit. In order to verify the reliability of this traveling mechanism, a prototype which works in the same way as the orbit-holding traveling mechanism was manufactured in this paper. And the experiment simulated the scanning equipment traveling along a orbit was completed, which showed that the traveling mechanism could travel along an orbit reliably when it weighed 24 kg and was exerted a 600N force. The prototype of traveling mechanism provided reliable evidence for the later traveling plan of scanning equipment.