Transient characteristics of fiber Bragg grating external cavity laser are presented. An effective photon lifetime is introduced to describe the photon behavior in external cavity condition. Simulation results show that the length of external cavity should be shorter than 4 cm in order to ensure the modulation frequency to be larger than 2.5 GHz. Single mode emission of an external cavity laser was obtained experimentally at 1 GHz modulation with side mode suppression ratio better than 40 dB, 3 dB dynamic line width about 0.1 nm, and 20 dB dynamic line width about 0.3 nm.