The experimental investigation on the interaction length for getting the optimum diffraction of the multi-order acousto-optic diffraction is introduced.Measurement results show that best acousto-optic interaction length is closely related to audio frequency，and it is not almost relate to the wavelength and power of the light.When audio frequency is lower，the interaction length became shorter.As audio frequency is about 9MHz，the interaction length is about 8mm，it shrinks to about 4mm at 6MHz of audio frequency.An acousto-optic Q-switch apparatus working with pure water is designed，and a total diffractive efficiency of about 98% is obtained under the condition of acoustic frequency at 9MHz and acoustic power at 3.4W.