Abstract: Experimental researches on the coherent properties of instability waves in the mixing layer of a planar jet under the acoustic excitation are proceeded through the measurement of the coherent time scale. It demonstrates that the coherent time scale keeps a high level in the growing stage of coherent structures under the motivation of instability waves and then turning down abruptly at the breaking point where the large-scale eddies begin to shed downstream. It provides a criterion in the contour map of the fluctuating velocity amplitude distribution to distinguish where the instability waves or convection eddies become dominant with their particular expression of Lighthill stress.