The instantaneous spectral characteristics of semiconductor lasers during the tuning process, such as the instantaneous wavelength, tuning rate, power, line shape, and linewidth, affect the accuracy of optical precision measurements and coherent optical communication systems that use tunable lasers as their light sources. Nevertheless, a method that can measure their performance simultaneously has not yet been reported. The purpose of this paper is to provide a novel method for the measurement of the instantaneous spectral performance of a semiconductor laser using time-frequency analysis. We designed a short-delayed self-heterodyne measuring system, described the time-frequency distributions of a laser’s optical field and the beat signal, determined the relationship between a laser’s instantaneous spectral performance during tuning and the parameters of the beat signal, and obtained the time-varying optical power spectrum of a semiconductor laser under continuous injection current tuning. The proposed short-delayed self-heterodyne measuring system used a Mach-Zehnder interferometer (MZI) with an optical path difference (OPD) of 10 cm. The tunable semiconductor laser (FRL15DDR0A31-18950, Furukawa) was tuned using a sawtooth injection current with an amplitude of 20 to 120 mA and a frequency of 1 kHz. According to the principle of coherent measurement, we considered a beat voltage as a superposed oscillation signal consisting of three components: the DC voltage signal, the noise, and the pure beat signal. The DC signal is directly proportional to the power of the laser. The noise generated from the laser’s phase variation can be used to calculate the linewidth and the line shape of the semiconductor laser. The pure beat signal is a mono-component amplitude and frequency modulation (AM-FM) signal, which can be named the carrier; its frequency is closely related to the wavelength of the laser. Using the trend local mean decomposition method, the instantaneous power, wavelength, tuning rate, and linewidth of a semiconductor laser were confirmed simultaneously during the continuous tuning process. The beat signal cut off the relaxation section is an oscillation with an increasing trend in the voltage as the injection current increases from 60 to 115 mA, in this range of changes, the power grows linearly from 5.16 to 10.6 mW, the wavelength changed linearly from 1 579.2 to 1 579.6 nm during the tuning process, the tuning rate increased from 0.004 8 to 0.011 5 nm·mA-1 and the instantaneous linewidth of the semiconductor laser ranges from 852.55 to 954.95 kHz during the entire duration. The results indicated that the time-varying spectral performance of a semiconductor laser can be obtained more accurately and conveniently. This instantaneous spectral performance measurement method based on the short-delayed self-heterodyne measuring system and time-frequency analysis can help in precisely obtaining the instantaneous characteristics of a semiconductor laser during the tuning process and requires only a simple optical system. This method makes possible a deeper and more fundamental understanding of the dynamic workings of a tunable laser, and we believe it should be widely applied.