With the extensive application of 2.94 μm laser in medicine, the control of laser parameters are particularly important to improve medical safety, therefore, Q-switched operation of Er:YAG laser is necessary. With LiNbO3 crystal cut at Brewster’s angle acted as electro-optical Q-switching of the Er:YAG laser, at 5Hz repetition rate and pump energy of 92 J, giant pulses of 103～110 ns duration and 87 mJ energy were obtained. A free-running and Q-switched pulsed laser was used for the cavity preparation experiments of the dentine respectively. Using scanning electron microscopy, the samples showed that Q-switched laser is more efficient.