Active-active mode-locking laser is very strict to its cavity length, and any small changes may result in mode-locking failure. The usual way to adjust the cavity length easily leads to cavity mirror mismatch. In this paper, precision cavity length adjustment is studied and a new way to adjust the cavity length is reported, which doesn′t easily result in cavity mirror mismatch. The precision of cavity length adjustment and the effect to the laser pulse width are also studied. Moreover, it is used in active-active mode-locking laser and good result is obtained.