Abstract: The peak power is determined by the modulation frequency and pulse width with the average output power of laser settled, thus the tunability of pulse width has its value in practice.The thulium-doped Q-switched double-clad fiber laser was investigated. With optimum transmissivity of 65% the output power of 2 m laser was 3.9 W and the corresponding slope efficiency was 31%. The signal gate width was adjusted from 12 s to 18 s with the external signal generator. Pulse width was measured at modulation frequency of 30 kHz, 40 kHz and 50 kHz, and maximum peak power of 555.6 W was attained. The pulse width varied from 163.4 ns to 207.9 ns. The experiment result shows pulse width of laser can be effectively controlled by using external signal generator.