Abstract: The method of abnormal noise diagnosis of engine based on Symmetrized Dot Pattern(SDP) is introduced in this paper.The time domain waveform is transformed into polar diagram through the measured sound signals to judge whether the measured engine produces abnormal noise or not by using the correlation coefficient in a certain diagrams.Compared with the traditional diagnosis method(finding amplitudes differences in the time or frequency domain),it has the advantages in short-time analysis and visual discrimination.The experimental results show that applying SDP can identify the difference between the sound signals generated by normal and faulty engines to realize the diagnosis of abnormal sound.