Based on virtual instruments, the direct method of sound intensity acquisition was discussed. According to the principle of sound intensity test with double microphones, signals captured by sound intensity sensor were transformed in a data acquisition system. Then the real time sound intensity (time domain sound intensity) signals via digital filter were obtained. Finally, the active sound intensity (frequency domain sound intensity) signals was obtained by time average. Based on the method, the engine time domain sound intensity signals were converted to diagnostic index, the difference of characteristics among different working conditions of engine were enhanced, and the accuracy and validity of fault diagnosis were improved. Through sound intensity calibrator testing, the engine sound intensity measurement and diagnostic examination, the method proved correct and reliable. The new method of sound intensity acquisition, developed in this work, provides a new way for the inspection of engine working condition and fault diagnosis.