A power transformer is one of the main sources of noise in the power transmission and transformation system, and the accuracy of outdoor noise prediction directly affects noise evaluation and control. Based on the test results of the wall vibration phase and amplitude in different excitation frequencies, this paper concluded that the distribution of the model phase and amplitude were random when the excitation frequency reached up to the seventh modal frequency. Thus, the zoned format could be simplified from power transformer to point sound source. Further, the coherent image source model of the power transformer was established considering the interference effect among the infinite image sources caused by the multiple reflections of sound rays. Finally, the sound field of the power transformer groups was calculated using the proposed model, and the solution was compared with that using the ISO9613 and BEM model. The results show that the coherent source model can predict the trend of the spread of sound waves at different locations considering the phase information, and the calculated value was closer to the exact value than that of the ISO9613 model. The above research results validate the proposed theoretical model.