We have systematically analysed the duration of laser pulse generated by using intracavity self-injection for various cavity arrangements. With the ferrite transmission line technique a driver of high voltage pulse with continuously tunable duration was constructed. A particularly designed avalanche circuit drives the Pockels cell PC1 to perform both Q-switeh-ing and cavity dumping. Only changing the voltage pulse duration but not the cavity arrangement, a 3 MW single laser pulse output continuously and conveniently tunable from 1.3 nsec to 6 nsec was obtained form a Q-switched phosphate Nd: glass laser.