Abstract: A new theoretical explanation is proposed for the Michelson interference pattern under the extended light source.The light emitted by the extended light source，due to the reflection of the two mirrors，there are two virtual images of the corresponding light source.The light emitted by the two virtual images is transmitted on the screen with the light from the original light source through the beam splitter，the screen forms an equivalent interference.Through this interpretation，we can more clearly understand the process of the process of the circle，oval，curve，straight lines and other stripes.

Key words: extended light source, Michelson interferometer, interference pattern