Abstract: In this research, a new self-trigger ultrasonic passive localization system for partial discharges and its hardware and software realization have been described Either the;principle of ultrasonic trigger and on-line test or the useful algorithm of time delay estimation and passive localization is discussed theoretically. By means of experimental,and industrial operation,this system has been proved to have many special advantages, especially having important practical value in examination of electrical equipment,Principle and method are introduced in this paper as part I.