The instantaneous linewidth in the process of tuning is a very important parameter of laser, however only static linewidth in steady state could be obtained by all kinds of measurement at present. So we proposed a method of measuring instantaneous linewidth by time-frequency analysis based on local mean decomposition, and after its validity of extracting the instantaneous phase noise of simulation signal was proved, we set the heterodyne measurement system with ultrashort delay (about 0.5 ns) which came from 10 cm optical path difference. Finally, we not only extracted the time-frequency spectrum of the beat signal’s phase noise power, but also obtained the instantaneous linewidth of distributed-feedback diode laser under the pulse current for the first time so far as we Rnow.