Method for controlling cavity length of seed injection laser driven cavity and its laser

The invention provides a laser device and a method for controlling the length of a driven cavity of a seed injection laser. The control method is described as following: the back cavity mirror is adhibited to a piezoelectric ceramics, and a piezoelectric ceramics driving circuit applies a linear scanning voltage on the piezoelectric ceramics during each pumping pulse, and turns on a Q switch when the seed light transits the driven cavity thereby forming interference signal peak, and then emits single longitudinal mode Q-switched laser, meanwhile, a sequential control system executes feedback control to the delay time of scanning voltage initial time relative to pumping pulse initial time as soon as pulse finishing, so that each moment of turning on the Q switch is at the position same with that of the pumping pulse trailing edge. The probability that the invention can emit single longitudinal mode pulse is close to 100%, so that the energy stabilization between pulses can be greatly improved.