Based on the theory of lasing wavelength red shifting of semiconductor laser due to junction temperature rising in pulsed operation,the time-resolved spectrum was measured by means of scanning optical power during the pulse at different wavelengths with a Boxcar. The thermal relaxation time was calculated by the measured relation of peak moment of optical signal within the pulse varied with wavelength. A pulse semiconductor laser was measured,and the thermal relaxation time was obtained to be 1.2 ms.