This paper explores a new method of PD monitoring on the insulation of large motors， describes a real-time on-line PD monitoring system based on ultrasonic and visual instruments， and analyses its principles and functions. The propagation characteristics of the PD ultrasonic signal in the large motor stator are discussed. Also， an analysis is made of the propagation difference of the PD ultrasonic signals between the weak background noise during normal running of generators and strong mechanical background noise. Experimental results show that it is feasible to ultrasonically monitor winding insulation PD for a large motor， which is preparatory work for PD location.