For the positioning accuracy of ultrasonic array detection for partial discharge(PD) of power transformer ,an array of time delay library is established ,the basic idea of which is making the center element of array sensors as the origin of coordinate system, and then dividing the transformer model into several grids by spacing of 1cm, each grid intersection is assumed to put PD source and then calculate the time delay of each element of array sensors and form a library. Through the space search based on genetic algorithm, matching the real array of time delay with the array of delay library, and then find the one with highest similarity and put this point as a PD source location. A large number of experimental studies have shown that the errors of the method are within 4cm and the positioning results are satisfactory, proved the feasibility of this method.