In this paper, the method of gain switched distributed feedback semi conducter laser with Fabry Perot Resonator (FPR) filter to generate transform limited pulses is analyzed theoretically. The filter bands needed to make pulse satisfy transform limited condition for different sourses are obtained and the corresponding output pulse width and spectrum width are calculated. The transform limited pulse is generated experimentally. The influence of resolution of spectrum measuring instrument on measured result is analyzed, and the correction theory is derived.